ruby: slicc: change enqueue statement

ruby: slicc: change enqueue statement
As of now, the enqueue statement can take in any number of 'pairs' as
argument.  But we only use the pair in which latency is the key.  This
latency is allowed to be either a fixed integer or a member variable of
controller in which the expression appears.  This patch drops the use of pairs
in an enqueue statement.  Instead, an expression is allowed which will be
interpreted to be the latency of the enqueue.  This expression can anything
allowed by slicc including a constant integer or a member variable.