Functor application dispatch on argument

[toumix]

Every time we do Functor.__call__(self, other), we dispatch on self which then goes through the whole hierarchy of classes. It would be more efficient to dispatch on other instead, e.g.

Ob.__functor_call__ = lambda self, other: other.ob[self]
Box.__functor_call__ = lambda self, other: other.ar[self]
Arrow.__functor_call__ = lambda self, other: other(self.id(self.dom)).then(*map(other, self))
Functor.__call__ = lambda self, other: other.__functor_call__(self)