Simplify + and * on construction when the type is commutative and associative

[YingboMa]

See https://github.com/symengine/symengine/blob/master/doc/design.md#object-creation-and-is_canonical

It is way faster than what we are doing now.

[shashi]

Okay I'm down to do this now. I avoided it initially because the rules just used stuff like ~a + ~b on the RHS and that would again trigger canonicalization, now we use similarterm(+, ~a, ~b) so we should be good.

[shashi]

OK we need to restrict #154 to only when types are associative commutative. Also, promote the output types properly.