Trim memory usage of IReadOnlyPackage implementations.

These implementations are often backed by a Dictionary, and tend to live a long time after being loaded. Ensure TrimExcess is called on the backing dictionaries to reduce the long term memory usage. In some cases, we can also preallocate the dictionary size for efficiency.

OpenRA.Mods.D2k/PackageLoaders/D2kSoundResources.cs

@@ -54,6 +54,8 @@ namespace OpenRA.Mods.D2k.PackageLoaders
 						var length = s.ReadUInt32();
 						index.Add(name, new Entry(offset, length));
 					}
+
+					index.TrimExcess();
 				}
 				catch
 				{