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>

docs/mkdocs/docs/examples/basic_json_view__get_to.cpp

@@ -0,0 +1,28 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+#include <string>
+
+using json_document = nlohmann::json_document;
+using json_view = nlohmann::json_view;
+
+int main()
+{
+    json_document doc = json_document::parse(R"({"host": "db.example.com", "port": 5432, "ssl": true})");
+    const json_view config = doc.root();
+
+    // get_to() writes directly into existing variables -- handy for filling
+    // in the members of a struct one field at a time, without an
+    // intermediate value from get<T>() for each one
+    std::string host;
+    int port = 0;
+    bool ssl = false;
+    config["host"].get_to(host);
+    config["port"].get_to(port);
+    config["ssl"].get_to(ssl);
+    std::cout << host << ':' << port << (ssl ? " (tls)" : "") << '\n';
+
+    // the return value is a reference to the argument, so a call can be
+    // used directly in a larger expression
+    std::string other_host;
+    std::cout << config["host"].get_to(other_host).size() << '\n';
+}