Other changes:
- Get rid of * spec pattern (deprecated in iris trunk)
- Make freeable_sz opaque (and simplifiy some proofs because of this)
- Refact option_as_mut and cell
- Prove lft_glb_acc, for gettings tokens of an intersections from tokens of its components
- Add some support for the = operator for ints in the proof mode

Boxes for function types are part of the function type itself and we do not need to add them everywhere.

This reverts commit 04386c1c.

For consistency with how we call this elsewhere in Iris.

Cleaning : remove hints about [product2] from solve_typing (this type is only for internal use), and removed unnecessary unfold lemmas.

Typechecked lazy lifetime initialization. Also, switched from eauto to typeclasses eauto, which seems way less bugged (but I still found something strange...).