Is it desirable to have heterogeneous complex/real operators?

I never had heterogeneous real-complex addition or subtraction, and I've [removed](https://github.com/apple/swift-numerics/pull/11) the multiply and division operators because of type-inference issues discussed in the README. This is a minor inconvenience for some use cases, and we should investigate ways in which we could make them available--especially multiplication and division, which
- are used more frequently because of the natural interpretation of C as vector space over R
- would allow us to elide a lot of spurious arithmetic operations an get some perf wins

Start by reading the README discussion referenced in the issue and review the removed operators in pull request #11. Investigate whether heterogeneous real-complex addition, subtraction, multiplication, and division can be supported without the reported type-inference problems. Done means documenting a viable approach or a clear reason the operators cannot be added.

Written by the indexing model from the issue text.