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/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
@@ -1056,13 +1074,19 @@ 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