Add unit declarator to module declarations

As of Rakudo 2015.05, the `unit` declarator is required before using
`module`, `class` or `grammar` declarations (unless it uses a block).  Code
still using the old blockless semicolon form will throw a warning. This
commit stops the warning from appearing in the new Rakudo.

lib/Coroutines.pm

@@ -1,4 +1,4 @@
-module Coroutines;
+unit module Coroutines;
 
 my @coroutines;