INST_TT_HYPS instantiates types and terms in a theorem thm,
in the same way INST_TY_TERM does.
It also returns a list of the instantiated hypotheses, in the same order
as the uninstantiated hypotheses appear in the list hyp thm.
FAILURE
INST_TT_HYPS fails under the same conditions as INST_TY_TERM.