movement equivalence classes

These can be used as a basis for a bunch of pathing optimizations.

- Feasability of movement can be precomputed for each class, avoiding
  the worst-case pathfinding behavior

- A path could potentially be shared between all members of a class.
  This isnt necessarily the best path for any single unit, as it
  doesn't care about efficiency of movement across various terrain --
  but it would be a "reasonable" path that the whole group could take
  together.

- General pathing checks can be converted from intersection of sets of
  strings to a simple AND.

- Other, wilder things.

V2: be paranoid about too-long bit vectors.

OpenRA.Mods.RA/Move/Mobile.cs

@@ -72,6 +72,15 @@ namespace OpenRA.Mods.RA.Move
 			return TerrainSpeeds[type].Cost;
 		}
 
+		public int GetMovementClass(TileSet tileset)
+		{
+			/* collect our ability to cross *all* terraintypes, in a bitvector */
+			var passability = tileset.Terrain.OrderBy(t => t.Key)
+				.Select(t => TerrainSpeeds.ContainsKey(t.Key) && TerrainSpeeds[t.Key].Cost < int.MaxValue);
+
+			return passability.ToBits();
+		}
+
 		public readonly Dictionary<SubCell, PVecInt> SubCellOffsets = new Dictionary<SubCell, PVecInt>()
 		{
 			{SubCell.TopLeft, new PVecInt(-7,-6)},