In an effort to delete `Value.hashUncoerced`, generic instantiation has
been redesigned.  Instead of just storing instantiations in
`monomorphed_funcs`, partially instantiated generic argument types are
also cached.  This isn't quite the single `getOrPut` that it used to be,
but one `get` per generic argument plus one get for the instantiation,
with an equal number of `put`s per unique instantiation isn't bad.