Rewrite operator overloads using promotion rules

[mlubin]

It seems like we could cut down a lot of the operator overloading code and make Julia's generic operations work correctly by implementing promotion rules.

Ref JuliaLang/julia#6672

[IainNZ]

That sounds like it'd be a good thing - can you give an example? I'm having trouble thinking of what it would look like

[joehuchette]

It would be very nice if this works; that goes triply so for SDP operator code.

[mlubin]

Is this still useful? Did we come to the conclusion that promotion rules don't quite work for the case where a*b has a different type from a+b?

[mlubin]

Not much going on here. We'll reconsider if we find a new reason to refactor the operator overloads.