json_view: from_json() of the macros reads views directly (#5922)

get<T>() and get_to() of a view call T's from_json() with the view itself
when that from_json() accepts views but not an unrelated probe type, and
fall back to materialize() otherwise. The from_json() functions of the
NLOHMANN_DEFINE_TYPE_*, NLOHMANN_DEFINE_DERIVED_TYPE_* and
NLOHMANN_JSON_SERIALIZE_ENUM(_STRICT) macros accept basic_json values and
views; a derived type whose base reads only json reads the base part
through materialize(). The library's own from_json() overloads are not
called with views: from_json() of a view (and of the probe) only looks up
functions by ADL.

New public trait nlohmann::is_json_view<T>, and the member type
basic_json_type of basic_json_view. With duplicate keys, the members of
such a T are those the view's lookups find (the first ones).

bench_decode.cpp measures decoding into structs (4-6x faster than
materialize().get<T>() on twitter, citm_catalog, and synthetic records).

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

tests/benchmarks/json_view/README.md

@@ -59,6 +59,12 @@ An editable `json_document` edits in place; yyjson copies its immutable document
 (`yyjson_doc_mut_copy`); Boost.JSON and `json::parse` build mutable DOMs; simdjson cannot edit a document. All
 outputs are checked to describe the same value.
 
+`bench_decode.cpp` measures decoding into structs whose `from_json()` comes from the `NLOHMANN_DEFINE_TYPE_*` macros
+(twitter statuses, citm_catalog performances, and 20,000 synthetic records): `get<T>()` on a view, which reads the
+view directly, against `materialize().get<T>()`, `json::parse` plus `get<T>()`, and `get<T>()` on an already parsed
+`json` value. Only nlohmann/json takes part, as the other libraries have no comparable mapping; all engines are
+checked to decode the same values.
+
 Before anything is timed, all engines must accept each document and agree on the traversal: the number of values, the
 bytes of all strings and keys, and the sum of all numbers. All engines run interleaved in every round, and the best
 round is reported, as time and as a factor of the `json_view` time (below 1 means faster than `json_view`). Each timed