`Cow` equivalent in PyO3

I propose to add a type to support copy-on-write behavior just like the std::borrow::Cow. The Cow type in std is usually unwanted because a reference to a Python object is held though PyRef. Therefore something like the following will be useful:

enum PyCow<'a, B>
where
    B: 'a + ToOwned + PyClass,
{
    Borrowed(PyRef<'a, B>),
    Owned(<B as ToOwned>::Owned),
}

This is useful when a function accepts both a Python object or something that can be converted to the object. For example, if I have a rust-implemented Python-type Decimal, a function that accepts a either Decimal or a string can take this as input to prevent a copy.

(Not sure if this is useful for other people, or if it's compatible with the new Bound API or not... Just post the idea here for discussion)

The issue proposes a PyCow type for copy-on-write handling of Python objects, using PyRef and considering compatibility with the Bound API. Start by reviewing those APIs and std::borrow::Cow; done would require a resolved design and maintainer agreement on whether and how this belongs in PyO3.