Add json_document and json_view: node index, parser, and document

Add json_document and json_view, a read-only, zero-copy index of a
JSON text, as the first public slice of the zero-copy view (#5295).

A parse produces a flat array of 16-byte nodes in document order,
one per value and one per object key. Strings stay in the source
text; escaped strings are decoded into an arena. Integers are
converted while their digits are in the cache; floats keep only
their digit layout and are converted on read. Containers store the
size of their subtree, so a reader can step over one in constant
time. A document makes a handful of allocations, however many
values it has.

The parser accepts exactly what json::parse accepts, with every
combination of ignore_comments and ignore_trailing_commas, with and
without a trailing NUL, and under JSON_STRICT_NUL_HANDLING. It is
portable C++11 and does not depend on byte order.

basic_json_document adds parse, parse_copy, accept, read (reuses a
document's memory), root, is_discarded, source, owns_source,
node_count, memory_usage, and shrink_to_fit. basic_json_view adds
type, the is_* queries, operator bool, size, empty, materialize,
and source_offset. A parse error throws the same exception
basic_json::parse would throw for the same input, message and
position included.

detail::abi_config keeps JSON_STRICT_NUL_HANDLING and
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON readable after json.hpp
undefines them, in the ABI namespace so they always match the
basic_json in use.

A NUL byte that ends a // comment is the end of the input, as in
parse() since #5696.

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

docs/mkdocs/docs/api/basic_json_view/materialize.md

@@ -0,0 +1,69 @@
+# <small>nlohmann::basic_json_view::</small>materialize
+
+```cpp
+BasicJsonType materialize() const;
+```
+
+Builds the `BasicJsonType` value of this subtree: the value [`BasicJsonType::parse()`](../basic_json/parse.md) would
+have produced for the same source text, allocated for the first time by this call.
+
+## Return value
+
+The `BasicJsonType` value of this subtree, or a discarded `BasicJsonType` value (`#!cpp BasicJsonType(value_t::discarded)`)
+if the view is [discarded](is_discarded.md).
+
+## Exception safety
+
+Strong guarantee: if an exception is thrown, there are no changes to the view or the document it refers to (nothing
+about either is mutated by this function).
+
+## Exceptions
+
+May throw `#!cpp std::bad_alloc` (via `BasicJsonType`'s allocator) if constructing the result fails.
+
+## Complexity
+
+Linear in the size of the subtree.
+
+## Notes
+
+`materialize()` replays the subtree through the same SAX builder [`BasicJsonType::parse()`](../basic_json/parse.md)
+uses internally, so the result matches it exactly -- including, for an object, that a repeated key keeps only its
+last value. The replay is iterative, so it is not limited by the call stack the way a naive recursive conversion
+would be; the JSON nesting depth is limited only by available memory, as for `BasicJsonType::parse()` itself.
+
+Unlike parsing with [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) enabled, the values
+produced by `materialize()` do not carry source positions: there is no lexer run during the replay to record them.
+
+Calling `materialize()` on the same view repeatedly builds a new, independent `BasicJsonType` value each time; it
+never caches the result.
+
+## Examples
+
+??? example
+
+    The example below skips messages that are not useful -- a discarded value, or an empty array -- using only
+    [`is_array()`](is_array.md) and [`empty()`](empty.md), and calls `materialize()` only for the messages that are
+    actually used, so no `BasicJsonType` value (and none of its per-element allocations) is ever built for the
+    skipped ones.
+
+    ```cpp
+    --8<-- "examples/basic_json_view__materialize.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/basic_json_view__materialize.output"
+    ```
+
+## See also
+
+- [root](../basic_json_document/root.md) - the view of a document's root value
+- [`BasicJsonType::parse`](../basic_json/parse.md) - build a `BasicJsonType` value directly from a JSON text
+- [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) - source positions on parsed values (not
+  produced by `materialize()`)
+
+## Version history
+
+- Added in version 3.13.0.