[Eva] Changes the type of cvalue builtins.

- [cacheable] is given at the registration of a builtin, and not separately
  for each result of the builtin.
- Removes the offsetmap for the arguments: no builtin were using it anyway.
- Returns a Cvalue.V.t for the result instead of an offsetmap.
- Builtins can also return:
  + only the value of the result, in which case the post-state from the
    interpretation of the specification is used with the resulting value
    computed by the builtin.
  + only the post-states for functions with no froms dependency (other than the
    result and arguments) and no write of local variable addresses.

Simplifies some builtins accordingly.