Remove external dependencies from hash table implementation.

The hash table implementation shouldn't have to know about MUSH-specific
types like dbrefs. Future efforts will focus on removing these
dependencies from the slab allocator and several other areas. The end
result is that it will be easier to write test harnesses for core
components of Penn to make it easier to verify that patches don't
introduce regressions.

git-svn-id: https://pennmush.googlecode.com/svn/trunk@1770 ba372814-4f39-11de-9ad6-1127a62b9fcd