Add element access, iteration, values, and JSON pointers to json_view

Give basic_json_view the read-only access functions of basic_json:
operator[] and at() with keys and indices, front()/back(), find(),
contains(), count(), begin()/end() and cbegin()/cend(), items()
with structured bindings from C++17 on, and type_name().

Exceptions have the ids and messages of the const functions of
basic_json. Where basic_json has undefined behavior the view
answers safely: operator[] with a missing key or an out-of-range
index returns a discarded view, and front()/back() of an empty
container throw invalid_iterator.214. Objects are iterated in
document order, and all members are visited; duplicate-key lookups
find the first member (as yyjson and simdjson do), while parse(),
materialize(), and the map conversions keep the last value, as
parse() does. Keys of up to 16 bytes are compared with two
overlapping loads.

Add value conversions: get<T>()/get_to() for arithmetic types,
bool, nullptr_t, strings (std::basic_string copied,
string_view_t without a copy), BasicJsonType, views, std::vector,
and maps with string keys; get_string() for the string without a
copy; number_token() for the number exactly as written in the
source; value() with keys and JSON pointers; and operator[]/at()/
contains() with JSON pointers. Everything else, including types
with from_json(), goes through materialize() of that subtree.
get<T>() of arithmetic types is inlined down to the conversion, so
reading an integer needs no call.

detail::json_pointer_access exposes a pointer's reference tokens
to code outside basic_json.

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

include/nlohmann/detail/json_pointer.hpp

@@ -31,6 +31,11 @@
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 
+namespace detail
+{
+struct json_pointer_access;
+}  // namespace detail
+
 /// @brief JSON Pointer defines a string syntax for identifying a specific value within a JSON document
 /// @sa https://json.nlohmann.me/api/json_pointer/
 template<typename RefStringType>
@@ -43,6 +48,8 @@ class json_pointer
     template<typename>
     friend class json_pointer;
 
+    friend struct detail::json_pointer_access;
+
     template<typename T>
     struct string_t_helper
     {
@@ -1172,4 +1179,18 @@ inline bool operator<(const json_pointer<RefStringTypeLhs>& lhs,
 }
 #endif
 
+namespace detail
+{
+/// the reference tokens of a json_pointer, for code that resolves pointers
+/// without a basic_json value (such as the zero-copy view)
+struct json_pointer_access
+{
+    template<typename RefStringType>
+    static const std::vector<typename json_pointer<RefStringType>::string_t>& reference_tokens(const json_pointer<RefStringType>& ptr) noexcept
+    {
+        return ptr.reference_tokens;
+    }
+};
+}  // namespace detail
+
 NLOHMANN_JSON_NAMESPACE_END