Tracking issue for associated functions and consts of `std::char`.

[eduardosm]

The std::char module defines functions that could be instead associated methods of the char type (e.g., decode_utf16). Making them associated to the char type allows to use directly char::method instead of std::char::method. Similarly, the std::char::MAX and std::char::REPLACEMENT_CHARACTER could also be associated constants (like done with integer constants in #68952).

[eduardosm]

I can help to implement to do this. This is what I would do:

• Move the functions and constants into impl char and make them unstable (with features assoc_char_funcs and assoc_char_consts).
• Implement the non-associated versions on top of the associated ones.

[eduardosm]

Implement the non-associated versions on top of the associated ones.

Given the module structure inside the core::char module, I think it would be better to implement the associated functions on top of the other functions.

[eduardosm]

This is currently the issue pointed as tracking issue for the new features added in #71854, so this issue should be reopened or a proper tracking issue should be created.