Add editable json_documents: set, push_back, insert, and erase

basic_json_document gets a second template parameter, Editable
(false by default), plus the aliases json_editable_document,
json_editable_view, ordered_json_editable_document and
ordered_json_editable_view.

Editable documents can change values and structure without
rewriting the source text: set()/push_back() on values, keys,
array indices and JSON pointers; insert() before an array
element; erase() of an object key, array index or JSON pointer.

New values and element sequences go into edit storage that the
document owns and never moves, so views keep referring to their
value across edits and a parsed node never moves. Read-only
documents walk the plain node array and are unaffected.

Strings are checked for UTF-8 on entry, so dump() of an editable
document never throws type_error.316. Binary values cannot be
stored (type_error.319).

A seeded differential test applies random edits to an editable
document and to the equivalent ordered_json and compares both
after every step.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/home/architecture.md

@@ -194,8 +194,8 @@ packet-beta
 
 | Bytes | Field   | Type       | Contents                                                                                                                      |
 |-------|---------|------------|-------------------------------------------------------------------------------------------------------------------------------|
-| 0     | `kind`  | `uint8_t`  | the type, numbered as [`value_t`](../api/basic_json/value_t.md): 0 null, 1 object, 2 array, 3 string, 4 boolean, 5 signed integer, 6 unsigned integer, 7 float |
-| 1     | `flags` | `uint8_t`  | bits 0-1: where a string's bytes are (0: the source text, 1: the buffer of decoded strings, for strings with escapes); bit 2: the value of a boolean |
+| 0     | `kind`  | `uint8_t`  | the type, numbered as [`value_t`](../api/basic_json/value_t.md): 0 null, 1 object, 2 array, 3 string, 4 boolean, 5 signed integer, 6 unsigned integer, 7 float; 10 for a link (see below) |
+| 1     | `flags` | `uint8_t`  | bits 0-1: where a string's bytes (or a number's token) are (0: the source text, 1: the buffer of decoded strings, for strings with escapes, 2: the edit buffer); bit 2: the value of a boolean; bits 3 and 4: moved and new (see below) |
 | 2-3   | `extra` | `uint16_t` | numbers: the number of integer digits (low byte) and fraction digits (high byte), 255 for more; objects: the number of their key hash index (1-based), or 0; otherwise 0 |
 | 4-7   | `off`   | `uint32_t` | where the value starts: the first byte after a string's opening quote (or its position in the buffer of decoded strings), the first byte of a number or literal, the bracket of an array or object |
 | 8-11  | `len`   | `uint32_t` | strings: the length after decoding; floats and literals: the length of the token; arrays and objects: the number of elements |
@@ -245,6 +245,21 @@ flowchart LR
 All `flags` are 0. The integer's bytes 8-15 hold its value, 1; its `extra` says it has one digit. The float's `extra`
 says it has one integer and one fraction digit, and its `len` is that of the token `2.5`.
 
+Editable documents ([`json_editable_document`](../api/json_editable_document.md),
+[`detail/view/edit.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/view/edit.hpp) and
+[`detail/view/edit_storage.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/view/edit_storage.hpp))
+never write the source text and never move or resize the parsed index, so views stay valid while the document is
+edited:
+
+- A new scalar is written over its node. Its text (a string, or the token of a number as `dump()` writes it) goes to
+  the edit buffer, which `flags` bits 0-1 then name.
+- An array or object whose elements change gets the flag *moved* (bit 3): its elements then live in a separate
+  sequence (a header node, then the entries), whose number is in `off`. The entries are links (`kind` 10), whose bytes
+  8-15 hold the address of the value's node, so values never move.
+- A node written by an edit gets the flag *new* (bit 4): it has no position in the source text.
+- Views of read-only documents compile without any of this: how views walk the index is a template parameter
+  (`navigation<Editable>`).
+
 ## Input adapters
 
 Input is read via **input adapters** that abstract a source. Every input adapter provides this interface:

docs/mkdocs/docs/home/exceptions.md

@@ -812,6 +812,24 @@ does not list an enumerator and it is therefore converted like the first listed
     [json.exception.type_error.318] duplicate object key 'red'
     ```
 
+### json.exception.type_error.319
+
+[`basic_json_document::set`](../api/basic_json_document/set.md) and
+[`basic_json_document::push_back`](../api/basic_json_document/push_back.md) can store any `basic_json` value except
+a binary one: a `json_document` has no representation for [binary values](../features/binary_values.md), which only
+ever arise from parsing a binary format or from an explicit [`json::binary`](../api/basic_json/binary.md) value, not
+from JSON text.
+
+!!! failure "Example message"
+
+    ```
+    [json.exception.type_error.319] cannot store a binary value in a json_document
+    ```
+
+!!! note
+
+    This exception was added in version 3.13.0, together with editable [`json_document`s](../features/json_view.md).
+
 ### json.exception.type_error.321
 
 A discarded value (one created by [`parse()`](../api/basic_json/parse.md) with a callback that returns `false` for the
@@ -1066,13 +1084,19 @@ MessagePack's ext type and BSON's binary subtype are each stored in a single byt
 
 [`basic_json_document::parse()`](../api/basic_json_document/parse.md) and the other parsing functions of
 [`basic_json_document`](../api/basic_json_document/index.md) index a value's position in the source text in 32 bits,
-so they do not support an input of 4294967280 bytes (4 GiB minus 16 bytes) or more.
+so they do not support an input of 4294967280 bytes (4 GiB minus 16 bytes) or more. The same 32-bit limit applies to an **editable** document's own
+storage: [`set`](../api/basic_json_document/set.md) and [`push_back`](../api/basic_json_document/push_back.md) throw
+this exception once the strings and number tokens written by edits reach 4 GiB in total, or once more than
+4294967295 arrays/objects have had an element set or appended to them.
 
-!!! failure "Example message"
+!!! failure "Example messages"
 
     ```
     [json.exception.out_of_range.416] input of 4294967280 bytes or more is not supported by json_document
     ```
+    ```
+    [json.exception.out_of_range.416] edits of 4 GiB or more are not supported by json_document
+    ```
 
 !!! note