Fix Warnings

[jkyang92]

This fixes a few compile warnings coming from mathicgb. Here's a summary of the less straightforward changes:

1. std::move is counterproductive when used in a return, returns already try to use the move constructor/assignment when possible.
2. When we provide a copy constructor, we should provide an assignment operator.
3. MATHICGB_ASSERT and friends should always give a non-empty expression, the ((void)0) trick is the same thing the standard assert does