Split up OPAM packages into separate ones, port to Jbuilder

• Split up OPAM packages into multiple independent ones. We now
  have a standalone cstruct, and then separate cstruct-lwt,
  cstruct-async, cstruct-unix packages, and a ppx_cstruct
  package for the syntax extension. Transitional findlib packages
  with the old scheme are available, but now packages should migrate
  to using cstruct-async instead of cstruct.async for example.
  This has the added benefit of the OPAM package names now matching
  the findlib names. (#138 by @avsm @rgrinberg).

• Port build to jbuilder.
  See the README for local development instructions.

• Ensure that check_alignment only takes a non-zero argument
  for alignment (#143 #145 by @cfcs @avsm).