Support creation of rust::Str from consteval utf8 checked string literal

[wsxarcher]

This change adds a C++20-only path for constructing rust::Str from UTF-8 string literals. The _utf8 literal validates the input at compile time using consteval function and then calls the unsafe version of constructing a str from rust to avoid double checking it (https://doc.rust-lang.org/std/str/fn.from_utf8_unchecked.html).

This is useful for creating literal Str without utf8 runtime checking overhead.

It can be especially useful in non exception code such as Chromium as extra utf8 check would have to be done outside the cxx library as a potential failure will result in abort, bringing the checks for static literals from 2 to 0.

Working example:

rust::Str var{u8"Valid👍"_utf8};

This will not compile:

rust::Str var{u8"Invalid\xff"_utf8};