Improve ENSURE-METHOD

[robert-strandh]

When ENSURE-METHOD finds that no generic function with the right name
exists, it calls ENSURE-GENERIC-FUNCTION, but it calls it with the wrong
lambda list. A function is needed that computes a congruent lambda list for
the generic function, from the specialized lambda list that is given.

Furthermore, if the generic function does exist, there is currently no verification
that the lambda list of the generic function and the specialized lambda list are
congruent. Code needs to be added for this verification.