feat(set_theory/game/pgame): Tweak `pgame.mul` API (#13651)
feat(set_theory/game/pgame): Tweak `pgame.mul` API (#13651)

We modify the API for `pgame.mul` in two ways:

- `left_moves_mul` and `right_moves_mul` are turned from type equivalences into type equalities.
- The former equivalences are prefixed with `to_` and inverted.





leanprover-community/mathlib,leanprover-community/mathlib