Avoid some allocations during loading.

- In FieldLoader, cache boxed bools and some boxed ints.
- In FieldLoader, presize collections when parsing a List, HashSet or Dictionary.
- In FieldLoader, don't allocate a list of missing items until required.
- In FieldLoader, when a string value is passed, avoid wrapping this in a MiniYaml object by allowing both strings and yaml to be passed in the GetValue overload that does the real work.
- In Animation, avoid allocating no-op actions.
- In VxlReader, use EnsureCapcity to better size the Dictionary.
- In VxlReader change VxlElement to a struct.
- In Locomotor, presize TerrainSpeeds dictionary.

OpenRA.Mods.Common/Traits/World/Locomotor.cs

@@ -84,8 +84,9 @@ namespace OpenRA.Mods.Common.Traits
 
 		protected static object LoadSpeeds(MiniYaml y)
 		{
-			var ret = new Dictionary<string, TerrainInfo>();
-			foreach (var t in y.ToDictionary()["TerrainSpeeds"].Nodes)
+			var speeds = y.ToDictionary()["TerrainSpeeds"].Nodes;
+			var ret = new Dictionary<string, TerrainInfo>(speeds.Count);
+			foreach (var t in speeds)
 			{
 				var speed = FieldLoader.GetValue<int>("speed", t.Value.Value);
 				if (speed > 0)
@@ -98,6 +99,7 @@ namespace OpenRA.Mods.Common.Traits
 				}
 			}
 
+			ret.TrimExcess();
 			return ret;
 		}