Use CommutativeMulNumber everywhere it is needed (and no where it isn't)

[sethaxen]

Currently, rules for * are constrained to CommutativeMulNumber types. This is unnecessary, because only two numbers at a time are multiplied, so commutation is never needed. These constraints should be relaxed.

Conversely, the scalar rule for ^ requires commutativity but makes no such type constraint. It should be limited to CommutativeMulNumber.

There may be other cases.