Missing math functions for complex numbers

[ChrisKitching]

Functions such as exp() for complex (from <complex>) are missing.

Would a PR be appreciated, or were these omitted for some horrifying reason I haven't discovered yet? :D

[Morwenn]

As often mentioned in issues and pull requests (and that may answer some of the other questions you might have), this project was a toy project before all that I almost only maintain when people open issues or pull requests that aren't too hard to understand: I don't even have sufficient maths or CS knowledge to implement some of the functions contributed by other users. That's the main reason why exp and other functions are missing.

If you do provide a PR it will sure be appreciated because I most likely won't implement the missing functions myself :)