json_view: the library's std conversions read views directly (#5923)

* json_view: the library's std conversions read views directly

The library's from_json() overloads (containers, maps, std::array, C
arrays, pairs, tuples, optional, valarray, forward_list, enums, binary,
paths, and scalars) accept views: their traits use the view's
basic_json type, and the few accesses that differ (get_ptr) go through
small helpers. from_json() of a view now finds them; only the probe
still looks up by ADL only. get<T>() of a view thus converts these types
without materialize(), with the same values and exceptions; from an
object, a repeated key keeps the last value, as parse() does.

The view keeps its direct paths for vectors of the types it converts
directly (faster than from_json()) and for containers of views; its
other vector and map paths are replaced by from_json(). For basic_json,
std::valarray and std::forward_list are read in one pass (40 % and 3.5 %
fewer instructions); all other conversions are unchanged.

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

* json_view: enable_if_not_view_t instead of '!' in the from_json() templates

astyle spaces out the template argument list when it contains a negation;
the alias spells the condition with negation<> and leaves the lines alone.

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

* json_view: compact the template parameter lists astyle had spaced out

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

---------

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

docs/mkdocs/docs/api/basic_json_view/get_to.md

@@ -9,8 +9,7 @@ Converts the value to `T` and stores it in `v`, in one of three ways:
 
 - For the types that [`get<T>()`](get.md) converts directly -- arithmetic types, `#!cpp bool`,
   `#!cpp std::nullptr_t`, [`string_view_t`](index.md#member-types), `#!cpp std::basic_string`, `BasicJsonType`,
-  `basic_json_view`, `#!cpp std::vector`, and `#!cpp std::map`/`#!cpp std::unordered_map` with string keys -- `v` is
-  assigned: `#!cpp v = get<T>()`.
+  `basic_json_view`, vectors of these, and maps of views -- `v` is assigned: `#!cpp v = get<T>()`.
 - For a type whose `from_json()` accepts views (see [`get<T>()`](get.md)), that `from_json()` is called with the
   view and the existing `v`, without building a `BasicJsonType` value.
 - For every other type, the subtree is built into a `BasicJsonType` value and `#!cpp materialize().get_to(v)` is