[xla:ffi] Added support for token-typed arguments and results
[xla:ffi] Added support for token-typed arguments and results

This change is necessary to migrate the custom calls behind
jax.*_callback APIs.

I decided to encode tokens as scalar buffers with void storage type. Another
possibility was to introduce a dedicated argument/return type for tokens
and change all the internals to allow both buffers *and* tokens, but I thought
the extra complexity is not worth it.
