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>

tests/benchmarks/CMakeLists.txt

@@ -39,3 +39,10 @@ target_compile_features(json_benchmarks PRIVATE cxx_std_11)
 target_link_libraries(json_benchmarks benchmark ${CMAKE_THREAD_LIBS_INIT})
 add_dependencies(json_benchmarks download_test_data)
 target_include_directories(json_benchmarks PRIVATE ${JSON_BENCHMARK_INCLUDE_DIR} ${CMAKE_BINARY_DIR}/include)
+
+# the json_document benchmarks, unless the header to benchmark predates json_view.hpp
+if(EXISTS "${JSON_BENCHMARK_INCLUDE_DIR}/nlohmann/json_view.hpp")
+    target_sources(json_benchmarks PRIVATE src/benchmarks_view.cpp)
+else()
+    message(STATUS "${JSON_BENCHMARK_INCLUDE_DIR} has no nlohmann/json_view.hpp: skipping the json_document benchmarks")
+endif()