Improved efficiency of startup methods.

- ShpReader will copy the input stream into memory just once rather than for every header.
- ShpReader.CopyImageData switched to use Array.Copy since that uses some unsafe magic for speed.
- In ActorInfo, cache a GetType call and prevent needless materialization in PrerequisitesOf.
- In ObjectCreator, cache type and ctor lookups since these are expensive and often repeated.
- Implement IReadOnlyDictionary<T, U> on Cache<T, U> to provide some supplementary functions.
- In TechTree.GatherOwnedPrerequisites, rearrange a Boolean 'and' expression to evaluate expensive functions later in the chain, and use ContainsKey to speed up name check.

OpenRA.Game/Primitives/ReadOnlyDictionary.cs

@@ -25,8 +25,8 @@ namespace OpenRA
 	{
 		int Count { get; }
 		TValue this[TKey key] { get; }
-		IEnumerable<TKey> Keys { get; }
-		IEnumerable<TValue> Values { get; }
+		ICollection<TKey> Keys { get; }
+		ICollection<TValue> Values { get; }
 
 		bool ContainsKey(TKey key);
 		bool TryGetValue(TKey key, out TValue value);
@@ -68,9 +68,9 @@ namespace OpenRA
 
 		public TValue this[TKey key] { get { return dict[key]; } }
 
-		public IEnumerable<TKey> Keys { get { return dict.Keys; } }
+		public ICollection<TKey> Keys { get { return dict.Keys; } }
 
-		public IEnumerable<TValue> Values { get { return dict.Values; } }
+		public ICollection<TValue> Values { get { return dict.Values; } }
 		#endregion
 
 		#region IEnumerable implementation