Make json_view.hpp pass include-what-you-use

Mark the includes that are needed by design (json.hpp, the macro
unscope, <tuple> for the tuple_size specialization), take size_t from
<cstring>, and drop the forward declaration of basic_json_document that
the friend declaration makes redundant.

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

include/nlohmann/json_view.hpp

@@ -24,19 +24,18 @@
 #ifndef INCLUDE_NLOHMANN_JSON_VIEW_HPP_
 #define INCLUDE_NLOHMANN_JSON_VIEW_HPP_
 
-#include <cstddef> // size_t
-#include <cstring> // memcpy, strlen
+#include <cstring> // memcpy, size_t, strlen
 #include <iterator> // distance, input_iterator_tag, iterator_traits
 #include <map> // map
 #include <memory> // unique_ptr
 #include <string> // string
-#include <tuple> // tuple_element, tuple_size
+#include <tuple> // tuple_element, tuple_size // IWYU pragma: keep
 #include <type_traits> // decay, enable_if, integral_constant, is_arithmetic, is_base_of, is_integral, is_same, remove_cv, remove_extent
 #include <unordered_map> // unordered_map
 #include <utility> // forward, move
 #include <vector> // vector
 
-#include <nlohmann/json.hpp>
+#include <nlohmann/json.hpp> // IWYU pragma: export
 
 // the view builds on internals of the library: both must be the same version
 #if NLOHMANN_JSON_VERSION_MAJOR != 3 || NLOHMANN_JSON_VERSION_MINOR != 12 || NLOHMANN_JSON_VERSION_PATCH != 0
@@ -58,9 +57,6 @@
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 
-template<typename BasicJsonType>
-class basic_json_document;
-
 /*!
 @brief read-only handle to one value of a basic_json_document
 
@@ -1074,6 +1070,6 @@ class tuple_element<N, ::nlohmann::detail::view::view_item<View>> // NOLINT(cert
 
 }  // namespace std
 
-#include <nlohmann/detail/view/macro_unscope.hpp>
+#include <nlohmann/detail/view/macro_unscope.hpp> // IWYU pragma: keep
 
 #endif  // INCLUDE_NLOHMANN_JSON_VIEW_HPP_