Syntax extension for building values of monoids

This is a syntax extension for OCaml to make building values of monoids easier. Assumes the existence of two operations in scope for some type t:

empty : t
(^^)  : t -> t -> t

ppx-monoid, triggered by the PPX extension point monoid, reinterprets the semicolon ; to mean the monoid operation ^^ and the unit expression () to mean empty.