add new is_constructible_* traits used in from_json

Add Meson related info to README

Add more information about how one can get a Meson wrap file.
Signed-off-by: Kostiantyn Ponomarenko <konstantin.ponomarenko@gmail.com>

Improve diagnostic messages for binary formats

This commit is the equivalent of #1282 for CBOR, MessagePack, and UBJSON.

is_compatible_* traits were used in from_json, but it made no sense
whatsoever.

It used to work because of non-SFINAE correctness + json_ref
unconstrained variadic template constructor.

SFINAE checks are becoming quite complex, we need a specification of
some sort describing:

* which concepts the library uses
* how the conversion to/from json works in detail

Having such a specification would really help simplifying the current
code (as well as having meaningful checks).

Fixes !1299

add constraints for variadic json_ref constructors

Fixes #1292

Improve diagnostic messages

Removed linter warnings

This is a follow-up to #1210.

Thirdparty benchmark: Fix Clang detection.

CMake 3.0+ refers to macOS’ Clang as AppleClang, which would fail a STREQUAL check. Fixed by changing it to MATCHES.

Fix warning C4127: conditional expression is constant

docs: Add additional CMake documentation