General cleanup of Complex and BigRational code imported from the F# PowerPack

This PR is a general cleanup of the Complex and BigRational code imported from the F# PowerPack. I've reformatted the code to "canonical" F# style guidelines and consolidated the `Complex.fsi` signature file into the implementation file (`Complex.fs`).

I did not change the implementations in any way, and all tests still pass. The goal behind these changes is to make the code easier to work with going forward, so I (or other F# developers) can start working to refine the existing implementation and eventually begin adding new features.