Move standard macros and compiler-generated over to rvalue promotion.

[eddyb]

Now that rvalue promotion is stabilized we can start using it instead of hacky inner statics (that show up in symbol tables, complicate future extensions, etc.), e.g. in panic! and format_args!.

[toidiu]

@eddyb havn't contributed to this repo yet. is this an appropriate issue for a new contributor to tackle? Sounds like a lot of find and replacing and and some understanding of macro/rvalue

[Mark-Simulacrum]

#44312 is already taking care of this issue. Maybe @eddyb could suggest an alternative issue for you to work on?

[eddyb]

I'd suggest looking at E-mentor as usual.