Log message:
Updated to 112.24.00, and converted to use mk/fetch/github.mk.

- Added syntax to use `Sexp.to_string_mach` rather than `Sexp.to_string_hum`.

  Added syntax `%{sexp#mach:<type>}`, which is like `%{sexp:<type>}`,
  except it calls `Sexplib.Sexp.to_string_mach` instead of
  `Sexplib.Sexp.to_string_hum`.

  In fact, you can put any identifier after the `sexp#` and it will call
  `Sexplib.Sexp.to_string_<that identifier>`; however, there are no other
  such functions right now.

Log message:
Add OCaml library custom-printf into wip/ocaml-custom_printf.

This library defines a syntax extension that allows the use of
user-defined string conversion functions in format strings.