Use rrule of KwFunc for Core.kwcall

[nmheim]

This seems to fix non-differentiable keyword arguments by constructing the KwFunc defined in Diffractor.

[oxinabox]

@Keno I think this does not have the same concerns that #266 has, but I am not sure. Because for reverse mode everything is already bad.

Should we do this, and reconsider it later as needed? It unbreaks a lot of tests

[Keno]

Same as in #266 I think in order for this not to make inference worse, the method should be split into kw and non-kw versions.