rewrite the ffi tutorial with snappy as an example

The code samples are xfail'ed because the buildbot won't have `libsnappy.so` and I don't want to add boilerplate to all the snippets (but they're directly from my snappy bindings so I'll just send a pull request whenever they break). It works really well as an example because it's tiny library and lets the caller manage all the buffers.

I think everything stated in the `Interoperability with foreign code` section is accurate but it deserves a thorough double-check :).