Export just the interleaved parsing API in Control.Proxy.Attoparsec.

- Deleted what was in Control.Proxy.Attoparsec and moved the interleaved
  parsing support there, since that will be the only interface available.

- Deleted the tutorial, since it's useless now and it should be
  rewritten.

- Deleted Control.Proxy.Attoparsec.Control since it's useless now.

- Moved most of Control.Proxy.Attoparsec.Types to
  Control.Proxy.Attoparsec.Internal.

- For the record, tests are still failing.