Add json_document and json_view: parse, accept, types, materialize

The public classes of the zero-copy view (#5295), in the new header
<nlohmann/json_view.hpp>:

- basic_json_document<BasicJsonType>: parse (borrowing contiguous byte
  inputs, owning rvalue strings, streams, and other inputs), parse_copy,
  accept, read, root, is_discarded, source, owns_source, node_count,
  memory_usage, shrink_to_fit
- basic_json_view<BasicJsonType>: type and the is_* queries, size, empty,
  materialize (the value parse() would produce, built by the same SAX
  handler), source_offset
- the aliases json_document, json_view, ordered_json_document, and
  ordered_json_view

A parse error throws the exception basic_json::parse would throw for the
same input: the library parser is run on the failing input, so messages,
positions, and exception ids are the same. Inputs of 4 GiB or more are
rejected with out_of_range.416.

The single header single_include/nlohmann/json_view.hpp keeps including
json.hpp; make amalgamate, check-amalgamation, include.zip, and release
handle it.

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

include/nlohmann/detail/view/macro_scope.hpp

@@ -37,6 +37,14 @@
     #define NLOHMANN_VIEW_NOINLINE
 #endif
 
+#if defined(__GNUC__) || defined(__clang__)
+    #define NLOHMANN_VIEW_NODISCARD __attribute__((warn_unused_result))
+#elif defined(_MSC_VER)
+    #define NLOHMANN_VIEW_NODISCARD _Check_return_
+#else
+    #define NLOHMANN_VIEW_NODISCARD
+#endif
+
 // exceptions as in json.hpp (JSON_NOEXCEPTION, JSON_THROW_USER)
 #if (defined(__cpp_exceptions) || defined(__EXCEPTIONS) || defined(_CPPUNWIND)) && !defined(JSON_NOEXCEPTION)
     #define NLOHMANN_VIEW_THROW(exception) throw exception