Rename htab functions; rewrite find to be generic.

The htab functions find_str and add_str have been renamed to include _ref in
the places they previously noted _str. These are intended to manage an htab
containing references to any data; string or not.

The htab_find function has been divided up. htab_find_core now handles actually
finding the correct node. htab_find and htab_find_ref are just outward facing
functions for retrieving a specific kind of data from the node, depending on
what the htab is used for.
Modify htab_add to have a generic core.
Fix bug where an outdate node is returned.

If htab_add_core added a node that happened to push the htab past its load
factor, it would return a pointer to where the added node was *before* the
rehash. Now it does not do this.
Merge pull request #28 from PaytonTurnage/master

Modify htab to have generic function names, and find to be nonredundant.