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.