109.52.00

- Added a `Macro` module, with `load_sexp*` functions that support
  file includes and templates.

  The following new syntaxes are supported:

  ```ocaml
  (:include filename)
  (:let f (arg1 ... argn) sexp1 ... sexpn)
  (:use f (arg1 valn) ... (argn valn))
  (:concat a1 ... an)
  ```
- Added support to `with sexp` for a subset of GADTs.

  The new support is for types that use existentially quantified
  variables or plain variants written with GADT syntax.

  Existentially quantified variables still have to be wrapped with
  `sexp_opaque` generate compiling code.
- Fixed a type error in the code generated by `with sexp` in some
  cases of variant inclusions.