Add Table with conversions between wrapper types

[konstin]

We should add a table to the documentation that explains with trait and method to use to convert one wrapper type to another. This is complementary to #374.

For example you use ToPyObject::to_object to convert &PySomeNativeType to PyObject and there's AsPyRef to convert PyObject and Py<T> to PyRef<T>s.

[pganssle]

You beat me to it, I was going to suggest the same thing.

I was thinking there should be an N x N grid, possibly with color-coded backgrounds to indicate things like "zero cost conversion" and whether the transformation is safe.

[davidhewitt]

I'm going to work on this as tidy ups for 0.12.