src/method-aggregate.lisp: `and' and `or' combinations return decisive value.

Rather than canonifying to an integer result, effective methods with
`and' and `or' method combination now return the decisive value -- which
means that they can be used with any scalar return type (so the type
restriction has been removed).

Most significantly, the `or' combination is very useful with pointer
returns: it returns the result of the most specific direct method which
returns a non-null value.