Function dt_idhash_destroy

* Destroy an entire identifier hash. This must be done using two passes with * an inlined version of dt_ident_destroy() to avoid referencing freed memory. * In the first pass di_dtor() is called for all identifiers; then the second * pass frees the actual dt_ident_t's. These must be done separately because * a di_dtor() may operate on data structures which contain references to other * iden