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.

Example:

let empty = "" and (^^) = (^) in
begin%monoid
  "hello";
  " ";
  "world"
end

is translated to:

let empty = "" and (^^) = (^) in
"hello" ^^ " " ^^ "world"

It is also possible to use concat or concatenate instead of
monoid:

let empty = "" and (^^) = (^) in
begin%concat   (* or 'concatenate' *)
  "hello";
  " ";
  "world"
end

See the test/test.ml file for more examples of usage, and for
examples of how the translation interacts with if then else and
match expressions.

The main use case for this syntax extension is for writing code that
generates HTML which consists of long sequences of concatenated bits
of HTML.