Add $.attr handles * and $attr handles ** to spec

'handles *' should only delegate if the $.attr can do the method.
'handles **' should always try to delegate to $.attr. This allows for
something like:

    class Bar { method baz { 42 } }
    class Foo {
        has Bar $.bar handles *;
        has @.bax handles **;
    }

So that Foo.baz will be delegated to $.bar, but anything else will be
delegated to @.bax