Substitute early-bound lifetimes when invoking object method.

Also, refactor previously fixed substitution for check_item and
check_method, moving both down the control flow into check_bare_fn.
Add test using early-bound lifetimes in trait generic parameters.
Placate `make tidy`.

(Note that the commit originating this change should be eliminated
before merge/PR anyway.)
Fix Repr impl for method::Candidate to include the method_ty.