Gate-array router.
This is an initial version tuned specifically for the AMCC 3500
gate-array, intended mainly to develop experience in gate-array
routing.  It is intended that this form the foundation for a
more general-purpose routing system integrated with the other
Magic routing tools.

Overall structure:
-----------------
    - Build netlist (just like in router module)
    - Channel generation:
      Differences from Magic router:
	- Channels are provided by the user rather than being
	  generated automatically.  There are commands of the
	  several forms:

	    garoute channel xlo ylo xhi yhi
		Define a general-purpose routing channel

	    garoute channel xlo ylo xhi yhi [horizontal | vertical]
		Define either a horizontal or a vertical
		river-routing region

	- Channels are cleared by
	    garoute clear

	- Terminals can appear inside river-routing channels.

    - Terminal pin assignment:
      Simply assign each pin to both sides of the channel, making
      certain that there isn't already a pin in the way.  Assign
      edge pins to the channel side that is nearest.  If pins are
      not already aligned, we will do stem generation; it's up to
      the user to leave enough room in the channels.

    - Global routing: