Deprecate `as_mut()` and provide an alternative

[ChayimFriedman2]

as_mut() cannot be used correctly, any usage will invoke UB. As the nomicon says:

• Transmuting an & to &mut is Undefined Behavior. While certain usages may appear safe, note that the Rust optimizer is free to assume that a shared reference won't change through its lifetime and thus such transmutation will run afoul of those assumptions. So:
  • Transmuting an & to &mut is always Undefined Behavior.
  • No you can't do it.
  • No you're not special.

Best reviewed commit-by-commit