support library for OCaml preprocessor type conversions

 The type-conv library factors out functionality needed by different
 CamlP4 extensions that generate code from type specifications.
 .
 CamlP4 extensions adding support for some trailing "with foo" (e.g.
 "with sexplib" provided by sexplib, and "with bin_io" provided by
 bin-prot) after type declaration can rely on type-conv.