Fixes to the formula freezing mechanism
* Renamed `FormulaFreeze` to `FormulaFreezer`
* Use `FormulaFreezer` directly in `freeze`/`thaw` instead of taking
  the detour through a `Formula` object.
* Create one `FormulaFreezer` per `Formula` and use that instead of
  creating a new one every time.
Still to do:
* Store freezing data somewhere else. HOMEBREW_CELLAR/frozen.yaml perhaps?
* Don't call `Formula#factory` inside `FormulaFreezer`, especially not when
  the `FormulaFreezer` is created by `Formula`.