Intoroduce PyComplex

[kngwyu]

For #224.
Now I implemented just 3 APIs for it, from_doubles, real, imag.
Some questions:

• We should implement FromPyObject to num-complex? Since num-complex is very portable crate, I think it's good.
• We should implement bindings for other APIs exposed when #[cfg(not(Py_LIMITED_API)]? Pesonally, I don't feel them so useful. Part of them are

PyAPI_FUNC(Py_complex) _Py_c_sum(Py_complex, Py_complex);
PyAPI_FUNC(Py_complex) _Py_c_diff(Py_complex, Py_complex);
PyAPI_FUNC(Py_complex) _Py_c_neg(Py_complex);
PyAPI_FUNC(Py_complex) _Py_c_prod(Py_complex, Py_complex);
PyAPI_FUNC(Py_complex) _Py_c_quot(Py_complex, Py_complex);
PyAPI_FUNC(Py_complex) _Py_c_pow(Py_complex, Py_complex);
PyAPI_FUNC(double) _Py_c_abs(Py_complex);

(https://github.com/python/cpython/blob/master/Include/complexobject.h), but we can do these operations in Rust side.

[konstin]

num-complex sounds like a good idea. I'd put it behind an off-by-default feature flag and mention it in the readme.

I think those functions should also be wrapped in rust. If we implement them using the Add, Sub, etc. traits we can use the normal +, -, etc. on PyComplex, which would be pretty neat.

Otherwise it's looking good!

[kngwyu]

Now ready to review 🙂

[konstin]

Thank you, this looks great!

Your contributions have all been really good, so I've given you commit access ✨