Change the definition of VgHashTable to not have pointer type.
This is (a) consistent with how the other containers are defined
and, more importantly, (b) allows the constification of the hash table API.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14639 a5019735-40e9-0310-863c-91ae7b9d1cf9