core::iter::order functions now take two types of iterators.

Previously the type signatures of the ordering functions in
`core::iter::order` took two iterators, but only if they were
the same type of iterator.  This commit loosens that restriction
and allows different kinds of iterators (but with the same type
of elements) to be compared.
auto merge of #14289 : TyOverby/rust/master, r=alexcrichton

Closes #14278.

Previously the type signatures of the ordering functions in `core::iter::order` took two iterators, but only if they were the same type of iterator.  This commit loosens that restriction and allows different kinds of iterators (but with the same type of elements) to be compared.