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.