The function relied on "List.map" which is not tail-rec. With enough
plevel, it provoked a stack overflow. I've simply fixed it by performing
the mapped computation inside the fold directly.

Uses a reference to a function provided by Eva to compute the dependency
of a term.

- Cabs2cil.is_boolean_result returns true if the expression is of type _Bool
- use standard result instead of custom expression for cast of a boolean to _Bool

and update is_null_expr and is_non_null_expr in a similar manner as the other ones

The name seems to imply that it will return Some for any constant integer expression,
not only for integer literals