Use a custom method-combination for definterface.

This should give better M-. support.

* swank-backend.lisp ([method-combinations] interface): New
(definterface, defimplementation): Use it.
Merge branch 'master' into interface-method-combination
Set :documentation of generic function in definterface.

* swank-backend.lisp (definterface):
* swank-backend.lisp: Whitespace cleanup.
Update ChangeLog.