This change doesn't actually introduce any Fiat code yet. It sets up the
directory structure to make the diffs in the next change clearer.
Change-Id: I38a21fb36b18a08b0907f9d37b7ef5d7d3137ede
Reviewed-on: https://boringssl-review.googlesource.com/22624
Reviewed-by: David Benjamin <davidben@google.com>
