Add Path.change_extension

Signed-off-by: David Allsopp <david.allsopp@metastack.com>

src/path.ml

@@ -411,3 +411,7 @@ let rm_rf =
     match Unix.lstat fn with
     | exception Unix.Unix_error(ENOENT, _, _) -> ()
     | _ -> loop fn
+
+let change_extension ~ext t =
+  let t = try Filename.chop_extension t with Not_found -> t in
+  t ^ ext

src/path.mli

@@ -101,3 +101,6 @@ val rmdir : t -> unit
 val unlink : t -> unit
 val unlink_no_err : t -> unit
 val rm_rf : t -> unit
+
+(** Changes the extension of the filename (or adds an extension if there was none) *)
+val change_extension : ext:string -> t -> t