Until now, doing a typecasting to or from a CRUST type was
considered licit, because there were some specific cases where
it would be needed. Unfortunately, there are much more cases
where this should not be allowed because it would be very
harmful.

This MR fixes this by showing a WARNING when a typecast is
done from a crust-type to a non-crust type or vice-versa.
This warning can be manually disabled in those specific cases
where it is needed (like when using MALLOC, REALLOC or FREE).

Fix #1