send/recv: Make send/recv records not effectively opaque

The Raw_app module is not exported, which means that a user previously
could not inspect the fields of a send/recv struct when using netkat as
an external library. This commit fixes that situation by explicitly
defining the record in Async_NetKAT.
Merge pull request #319 from frenetic-lang/send/recv

Make send/recv records not effectively opaque