Allow a directed channel array to be sliced.

This lets you write things like "[cs! FOR 5]", which is horrible; I
would prefer "[cs FOR 5]!", since then that doesn't imply that you can
do things like "cs![0] ! 0".

However, Tock now compiles and passes cgtest87 -- the first occam-pi
cgtest we've handled. :)

frontends/ParseOccam.hs

@@ -843,7 +843,7 @@ channel
 channel' :: OccParser A.Variable
 channel'
     =   do { m <- md; n <- try channelName; return $ A.Variable m n }
-    <|> maybeSliced channel A.SubscriptedVariable
+    <|> maybeSliced directedChannel A.SubscriptedVariable
     <?> "channel'"
 
 direction :: OccParser A.Direction