The problem was that we need to apply the substitution, so that the
formal lifetime parameters get replaced with (unifiable)
free-lifetimes that can actually be fed into the constraint solver.
Also, refactor code os that substitution for `check_item` and
`check_method`, moving both down the control flow into `check_bare_fn`.
----
Finally, there was another (similar) spot where we needed to
substitute early-bound lifetimes when invoking an object method of a
trait.
6b5ca8b1aa