Guarantee that use of standard operators is equivalent to `create`

[goerz]

For any conceivable use of an operator (*, +), we should guarantee (via a test) that the resulting expression is identical to one that is created via a create constructor. E.g.

1 * OperatorSymbol('a', hs=1) == ScalarTimesOperator.create(1, OperatorSymbol('a', hs=1))