Consistent typing for yul level return and revert

[d-xo]

yul level return has type word -> word -> unit, whereas revert has type forall a . word -> word -> a.

These two operations are basically the same thing from the pov of the type system so we should probably give them the same type signature.

[rodrigogribeiro]

Fixed by #202. The idea was to use the same polymorphic type for return and revert.