Mixed mode arithmetic

[MarcusRainbow]

Python has no pride at all. You can write a mixture of integers, floats and even complex numbers in the same arithmetic expression. Rust is far less forgiving.

To support whatever Python code is likely to appear, we need to automatically add Rust casts to different numeric types. Arguably this could be controllable with an option, as mixed mode arithmetic is bad, and we may want to flag up usage rather than acquiescing.