Issue 5910 dyna freeze

This rather sprawling branch refactors the borrow checker and much of the region code, addressing a number of outstanding issues. I will close them manually after validating that there are test cases for each one, but here is a (probably partial) list:

  - #4903: Flow sensitivity
  - #3387: Moves in overloaded operators
  - #3850: Region granularity
  - #4666: Odd loaning errors
  - #6021: borrow check errors with hashmaps
  - #5910: @mut broken

cc #5047

(take 3)