Gmp.mpz: Propagate CLEAR_ON_EXIT to bignums.

Creating a bignum from a String.secure()d string now causes
the bignum to be Object.secure()d.
Added a few OBJECT_CLEAR_ON_EXIT clears.

Images, String.Buffer objects and Gmp.mpz objects now honor the flag
for not only the object structure, but also the data in the object.
Removed some redundant debug that debugs that pike can call functions correctly.