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

This addresses @GenTiradentes' note at the end of #27. It's relatively small.

* htab_find_core is a static method that finds a node based on the key and returns a pointer to the node or NULL
* htab_ find calls this and returns the value of that node, or -1 if it was not found
* htab_find_ref calls this and returns the valptr of that node, or NULL if it was not found