isl_*_dim: return isl_size

isl_*_dim: return isl_size

This improves both error handling and consistency.
Most of the isl_*_dim functions used to return "unsigned",
while isl_local_space_dim, isl_aff_dim and isl_constraint_dim
returned "int".  Only isl_constraint_dim returned
a negative value on error.
Now, all isl_*_dim functions return isl_size and
they all return a negative value on error.

All callers have been adjusted to take into account
this possibly negative return value.
In particular, since there was no way for the functions
returning "unsigned" to indicate an error condition,
all callers were forced to check the arguments before the call
(with no guarantee of catching all errors) and can now
simply check the return value.

Outside callers need similar adjustments.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>