bindings: provide nicer "just use an fd/file" API

[cyphar]

At the moment, you need to convert any file objects into Handle in both of our bindings, which (while providing nice type-safety) makes it a little ugly to do basic stuff with libpathrs. Ideally we would have something akin to HandleRef/RootRef to avoid needing clone as well.

[cyphar]

We can just have something like rawpathrs or create a version of Root or Handle that doesn't do anything on Close (__del__/close) -- i.e., RootRef and HandleRef. Unfortunately that's not as ergonomic as in Rust (and even in Rust it's a little rough).

[cyphar]

Go also really could do with a one-shot pathrs_reopen wrapper. HandleFromFile does a wasteful dup.