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/module_cpp20/main.cpp

@@ -52,8 +52,13 @@ int main()
     std::ostringstream os;
     os << j << oj << lit;
 
+    // json_document / json_view (json_view.hpp): zero-copy parse of a literal
+    const nlohmann::json_document doc = nlohmann::json_document::parse(R"({"a": 1, "list": [1, 2, 3]})");
+    const std::size_t doc_size = doc.root().size();
+
     // use every result so the references cannot be optimized away
     return (a == 1 && last == 3 && b == 2 && lit.size() == 3
-            && m.size() == 1 && !dumped.empty() && !os.str().empty())
+            && m.size() == 1 && !dumped.empty() && !os.str().empty()
+            && doc_size == 2)
            ? 0 : 1;
 }