Various "cast to pointer from integer of different size"

Various "cast to pointer from integer of different size"
picked up by gcc -Wall (in Tru64 where pointers are 64 bits
wide but I32 (int) only 32 bits wide).

WARNING: the classnum and tagnum changes in Storable.xs
may not be wise, they may be breaking binary compatibility
(in 64-bit platforms), asked Raphael Manfredi about the changes.

p4raw-id: //depot/perl@10417