Having the hash table key change during iteration is bad, so make it
impossible. Nothing relied on the key type not being const. Nothing
needed a mutable value either, so make the whole pair const.
Also rename the iterator typedef to const_iterator.
(This is also necessary to be able to call the const version of
iterator_facade_base::operator->(). Nothing calls this yet, but
I'm adding a few calls to it in a dependent change.)
No behavior change.