feat: allow writing custom matchers

[anuragsoni]

This allows custom matchers so one can write things like:

type shape =
  | Point
  | Circle

let shape_of_string = function
  | "Point" | "point" -> Some Point
  | "Circle" | "circle" -> Some Circle
  | _ -> None
;;

let shape_to_string = function
  | Point -> "point"
  | Circle -> "circle"
;;

let shape = pattern shape_of_string "<shape>"

Now shape can be used like the other built in matches (int, int32, bool etc)