Auto merge of rust-lang#13707 - lowr:feat/move-const-to-impl, r=Veykril

Add `move_const_to_impl` assist

Closes rust-lang#13277

For the initial implementation, this assist:
- only applies to inherent impl. Much as we can *technically* provide this assist for default impl in trait definitions, it'd be complicated to get it right.
- may break code when the const's name collides with an item of a trait the self type implements.

Comments in the code explain those caveats in a bit more detail.

Author: bors