Remove snap_ty type from the prelude

[jhjourdan]

The snap_ty 't type is just isomorphic to 't, so there is no need to keep this type. Creusot should just remove this from generated files, and make inner and new the identity.

Expected outcomes:

• SMT solvers do not need to instantiate new_spec and inner_spec. These types create a trivial matching loop.
• One fewer polymorphic type that Why3 will not need to encode.