Destructible bridges

This is jokermatt999's terrain with me on the mapgen.  Might need a few tweaks (less resilient and drop more rebar--it's perfectly retrievable) but IMO it's a significant improvement on the current bridges.

1) If you destroy it, it reverts to deep water.  No blowing the bridge and getting dirt.
2) Slightly wider, so your Semi can fit.
3) Sidewalks, too, so it's OK to cross on foot.
4) Heavier-duty guide rails that don't revert to "inside", so no more magic-insideness when you get caught in a storm.
Git artifact here.