Allow to get ffi types from the rust API

[pbor]

I would like to use the pcsc-rs crate to implement a rust module which in turn exposes a C api.

In my case I would need to

1. Go from Context to SCARDCONTEXT
2. Go from ReaderState to SCARD_READERSTATE`

It would be great if there were into_raw and from_raw methods.

[bluetech]

Now that we explicitly reexport pcsc-sys, this sounds fine to me. Will accept a PR for this.