Documentation: Add a table mapping Python types to Rust equivalents

[pganssle]

It is often confusing trying to figure out how to write a #[pyfunction] signature that will match the Python signature you want (see #373, for example).

It would be great to have two lookup tables for all native types, one mapping the Python -> Rust, for writing the types of the function parameters, and another mapping Rust -> Python, saying the type of the object you'll get if you return a given type. So the first table would look like this:

Python	Rust
int	Any integer type (i32, u32, usize, etc)
list	Vec<T>, &PyList
tuple	Vec<T>, &PyTuple
str	String, &PyUnicode

The second one might look like this:

Rust	Python
Vec<T>	list
u8, u16, etc	int

I don't think it needs to be comprehensive, but at least for some of the basic types (and especially showing the preferred way to do things) would be super helpful.

[davidhewitt]

I'm working on this table, started it this evening and will finish it off maybe on Sunday.