Change RRG storage to keep (drive pt, direction) instead of (start, end)

[duck2]

The current expansion pruning in the router uses the drive point of every RRNode ((xlow ,ylow) for non-directional nodes). This check is inside a hot loop (in connection_router.cpp), so there may be performance benefits to store (xdrive, ydrive, dir) in the RRG storage, since the router will only fetch two values instead of three + a branch.

For bidir wires (xdrive, ydrive) can be just (xlow, ylow).