params: add a mechanism to the config system to specify cycles
Latencies can be specified as '2ns' or '1t' meaning 2 nanoseconds and
one "tick" respectively.  This patch adds '1c' which means 1 cycle.
The cycle is the clock cycle of the current object, so the current
object must have an object called clock that is a parameter of type
Clock.  Literally, '1c' means 1 * Self.clock.latency, so it's not
strictly necessary, but it sure is handy.