Add values and JSON pointers to json_view

basic_json_view gains get<T>(), get_to(), value() with keys and JSON
pointers, and operator[], at(), and contains() with JSON pointers, plus
two functions basic_json has no counterpart for:

- get_string(): the string without a copy (a string_view into the source,
  or into the decoded strings for strings with escapes)
- number_token(): the text of a number as it appears in the source

get<T>() converts arithmetic types, strings (also string_view_t),
std::nullptr_t, std::vector, maps with string keys, and views directly;
floats are converted from the digit layout recorded by the parser with the
library's conversion chain, so the values are bit-identical to parse().
Other types, including user types with from_json(), go through
materialize().

The exceptions are those of basic_json, message included. Where const
basic_json has undefined behavior (a missing key or an index out of range
with operator[] and a JSON pointer), the result is a discarded view;
value() returns the default wherever basic_json catches out_of_range, and
contains() never throws. Array indices of JSON pointers follow
json_pointer's rules (parse_error.106/109, out_of_range.404/410).

Tests compare the conversions of 2,000 generated documents, 20,000 float
tokens (double and float, bit for bit), and every JSON pointer of 1,000
documents with basic_json, and the exceptions for malformed pointers.

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

include/nlohmann/detail/view/errors.hpp

@@ -40,6 +40,12 @@ namespace view
     NLOHMANN_VIEW_THROW(invalid_iterator::create(id, msg, nullptr));
 }
 
+/// a parse error without a position (as those of json_pointer)
+[[noreturn]] NLOHMANN_VIEW_NOINLINE inline void throw_parse_error(int id, const std::string& msg)
+{
+    NLOHMANN_VIEW_THROW(parse_error::create(id, 0, msg, nullptr));
+}
+
 /*!
 @brief throw the exception BasicJsonType::parse would throw for this input