Skolem let fix

[rodrigogribeiro]

This PR fixes an issue in type checking Yul code and adds a test case suggested by @mbenke.

[mbenke]

I have some doubts about the error message:

$ esolc test/examples/cases/skolem-let.solc 
test/examples/cases/skolem-let.solc
Type not polymorphic enough! The annotated type is:
forall a . word -> a
but the infered type is:
word -> word
in:
forall a . function fromWord (x : word) -> a

 - in:forall a . function fromWord (x : word) -> a {
   let result ;
   assembly {
      result := x
   }
   return result;
}

...but x is in fact declared to have type a (which is misrepresented in the error message) and the problem is in result := x but I see it may be hard to report in a better way ("types $123 and word do not unify" probably would be even less helpful)

[mbenke]

Well done. Just minor complaints (feel free to ignore my rant about error message).