Fix composition of cstruct.ppx with other drivers

• ppx: remove an errant standalone initialiser that was messing up
  the composition of cstruct with other ppx_driver based ppx converters
  (most notably ppx_sexp_conv. If you are having trouble with using
  ppx_cstruct with other drivers, put a constraint on ppx_cstruct>=3.0.1.
  (#151 #150 #149 #148 via @djs55 @g2p @avsm @diml).
• ppx: also add a test case for cstruct and lwt working together, but
  this will not work until a lwt>3.0 release happens.
• Update opam rules to use jbuilder subst for version information
  in the distribution.
• Fix tests so that check_alignment expects a negative result.
• Add opam test target for core library.

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).

Fix linking regression introduced in 2.4.0

• fix missing ppx_tools_versioned dependency (#136, @et-def)

Support jbuilder and ppx_driver

Distribute the PPX extension so that it is compatible with Jbuilder.
ppx_cstruct is now distributed as both a library and a binary.
Findlib predicates are used to distinguish usage:

• the binary is used for toplevel and simple -ppx building
• the library is used for linking custom rewriters
• -package cstruct.ppx alone uses the binary for rewriting.
• -package cstruct.ppx -predicates custom_ppx,ppx_driver" is used to link the rewriter.

To use the PPX extension in jbuilder, just add:

  (libraries (cstruct))
  (preprocess (pps (cstruct.ppx)))

to your jbuild file. This may be renamed to ppx_cstruct in a
future release so that the PPX dependency is decoupled from the main
library, so this cstruct.ppx is intended to be transitional as it
is what was originally used.

More flexible OCaml version support

This release uses ocaml-migrate-parsetree (described here) in order to make it more independent to the base OCaml version. There should be no observable changes, but the internals are much cleaner now.

OCaml 4.05 PPX support

• Add support for OCaml 4.05 for the PPX extension.
• Docs: correct to description of shift function (#121 via @orbifx).

Minor updates

• Fix a memory leak in the exception printing code (#130 via @djs55)
• Appveyor CI fixes (#130 via @avsm)
• Fix typo in docstring (#117 via @yomimono)
• Fix opam base-unix dependency (#115 via @avsm)

Add to/of_bytes functions and support OCaml 4.04.0

• Add Cstruct.of_bytes/to_bytes. In common with the existing
  implementation, this relies on the representation of bytes and string
  being the same, which is true as of OCaml 4.04 and lower (#105 via @yallop).
• Support OCaml 4.04 (#111 via @gasche).

Zero out new buffers by default

• Make create zero out the new buffer. The new create_unsafe
  function can be used if you want to trade safety for speed.

Add Format-friendly hexdump_pp

Add hexdump_pp that uses the Format module. This works better with the Logs library than using hexdump_to_buffer, and also makes it easy to indent the hexdump (#100 via @talex5).