Provide names and pools when creating MiniYaml.

- Rename the filename parameter to name and make it mandatory. Review all callers and ensure a useful string is provided as input, to ensure sufficient context is included for logging and debugging. This can be a filename, url, or any arbitrary text so include whatever context seems reasonable.
- When several MiniYamls are created that have similar content, provide a shared string pool. This allows strings that are common between all the yaml to be shared, reducing long term memory usage. We also change the pool from a dictionary to a set. Originally a Dictionary had to be used so we could call TryGetValue to get a reference to the pooled string. Now that more recent versions of dotnet provide a TryGetValue on HashSet, we can use a set directly without the memory wasted by having to store both keys and values in a dictionary.

OpenRA.Game/Exts.cs

@@ -130,6 +130,20 @@ namespace OpenRA
 			return ret;
 		}
 
+		public static T GetOrAdd<T>(this HashSet<T> set, T value)
+		{
+			if (!set.TryGetValue(value, out var ret))
+				set.Add(ret = value);
+			return ret;
+		}
+
+		public static T GetOrAdd<T>(this HashSet<T> set, T value, Func<T, T> createFn)
+		{
+			if (!set.TryGetValue(value, out var ret))
+				set.Add(ret = createFn(value));
+			return ret;
+		}
+
 		public static int IndexOf<T>(this T[] array, T value)
 		{
 			return Array.IndexOf(array, value);