Merge branch 'bugfix/julien/ctype_to_gmp' into 'master'

Fix 2 bugs with integer casts

This MR fixes two bugs with integer casts:
- when generating a new variable corresponding to a cast ```(ty)t```, its type was the one of the translation of ```t```, not ```ty```
- for a cast ```(ty)t``` the implementation of the type system didn't compute the smallest interval from both operands. It might result in incorrectness of the generated code as reported in https://bts.frama-c.com/view.php?id=2231 (which is now fixed). I guess this patch also solves your gmp issue with SpecCPU Benchmark.

See merge request !78