Add images of json_documents: save() and load()

An image is a document stored so that loading it needs no
parsing: save() writes the node index, the text and the decoded
strings; the static load() reads an image written by save().

load() takes a pointer and size, a borrowed vector, or an owned
rvalue vector; the nodes are copied so they are aligned and can
be edited, while the text and decoded strings stay in the image.

image_check controls how much load() trusts the input: full
checks structure, bounds, strings and numbers, the parser's own
guarantees; bounds checks structure and bounds only; none skips
all checks, for images from a trusted source.

Layout is little-endian only ("NJVI" header, nodes, text, decoded
strings), following the idea of zero-copy formats such as
FlatBuffers and YaFF; the check follows FlatBuffers' Verifier.

New errors: parse_error.116 for a malformed image or a failed
check, type_error.320 for a discarded document or a big-endian
target.

A dedicated fuzzer and 6,000 seeded corruptions, checked under
ASan/UBSan, found and fixed two gaps: unchecked reserved header
fields, and unbounded null/boolean offsets that could make
dump() throw std::length_error.

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

docs/mkdocs/docs/home/architecture.md

@@ -260,6 +260,14 @@ edited:
 - Views of read-only documents compile without any of this: how views walk the index is a template parameter
   (`navigation<Editable>`).
 
+Images ([`save`](../api/basic_json_document/save.md) and [`load`](../api/basic_json_document/load.md),
+[`detail/view/image.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/view/image.hpp)) store
+the nodes as they are: a 64-byte header (the magic bytes `NJVI`, a format version, the sizes, and reserved bytes that
+must be zero), the nodes, the text, and the decoded strings. An edited document is first written in document order, as
+the parser would have written it (without links), and the numbers of the hash indexes are cleared, since `load`
+rebuilds the indexes. So a change of the node layout is a change of the image format: it must raise `image_version`,
+and `load` then rejects images of other versions (`parse_error.116`) instead of misreading them.
+
 ## Input adapters
 
 Input is read via **input adapters** that abstract a source. Every input adapter provides this interface:

docs/mkdocs/docs/home/exceptions.md

@@ -395,6 +395,23 @@ A UBJSON high-precision number could not be parsed.
     [json.exception.parse_error.115] parse error at byte 5: syntax error while parsing UBJSON high-precision number: invalid number text: 1A
     ```
 
+### json.exception.parse_error.116
+
+[`basic_json_document::load()`](../api/basic_json_document/load.md) rejected an
+[image](../features/json_view.md#images): either the bytes are not one [`save()`](../api/basic_json_document/save.md)
+could have written (too short, an unknown magic number or format version, or sizes that do not fit the buffer), or
+they are, but fail the requested [`image_check`](../api/basic_json_document/load.md#image_check).
+
+!!! failure "Example message"
+
+    ```
+    [json.exception.parse_error.116] parse error: invalid json_document image: the check failed
+    ```
+
+!!! note
+
+    This exception was added in version 3.13.0, together with [images](../features/json_view.md#images).
+
 ## Iterator errors
 
 This exception is thrown if iterators passed to a library function do not match
@@ -831,6 +848,26 @@ from JSON text.
 
     This exception was added in version 3.13.0, together with editable [`json_document`s](../features/json_view.md).
 
+### json.exception.type_error.320
+
+[`basic_json_document::save()`](../api/basic_json_document/save.md) cannot write an
+[image](../features/json_view.md#images) of a [discarded](../api/basic_json_document/is_discarded.md) document.
+[`save()`](../api/basic_json_document/save.md) and [`load()`](../api/basic_json_document/load.md) also throw this
+exception on a big-endian target, since the image format is little-endian only.
+
+!!! failure "Example messages"
+
+    ```
+    [json.exception.type_error.320] cannot save a discarded json_document
+    ```
+    ```
+    [json.exception.type_error.320] json_document images need a little-endian target
+    ```
+
+!!! note
+
+    This exception was added in version 3.13.0, together with [images](../features/json_view.md#images).
+
 ### 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
@@ -1088,7 +1125,9 @@ MessagePack's ext type and BSON's binary subtype are each stored in a single byt
 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.
+4294967295 arrays/objects have had an element set or appended to them. The same limit applies to an
+[image](../features/json_view.md#images): [`save()`](../api/basic_json_document/save.md) throws it if the node
+count, the text, or the decoded strings it would write would individually reach 4 GiB.
 
 !!! failure "Example messages"
 
@@ -1098,6 +1137,9 @@ this exception once the strings and number tokens written by edits reach 4 GiB i
     ```
     [json.exception.out_of_range.416] edits of 4 GiB or more are not supported by json_document
     ```
+    ```
+    [json.exception.out_of_range.416] images of 4 GiB or more are not supported by json_document
+    ```
 
 !!! note