Add __foreach_active statement to loop over active prog. instances.

For now this has the __ prefix, as an experimental feature currently only
used in the standard library implementation.  It's probably worth making
something along these lines an official feature, but I'm not sure if this
in its current form is quite the right thing.

lex.ll

@@ -1,5 +1,5 @@
 /*
-  Copyright (c) 2010-2011, Intel Corporation
+  Copyright (c) 2010-2012, Intel Corporation
   All rights reserved.
 
   Redistribution and use in source and binary forms, with or without
@@ -358,6 +358,7 @@ extern { RT; return TOKEN_EXTERN; }
 false { RT; return TOKEN_FALSE; }
 float { RT; return TOKEN_FLOAT; }
 for { RT; return TOKEN_FOR; }
+__foreach_active { RT; return TOKEN_FOREACH_ACTIVE; }
 foreach { RT; return TOKEN_FOREACH; }
 foreach_tiled { RT; return TOKEN_FOREACH_TILED; }
 goto { RT; return TOKEN_GOTO; }