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/ExternalMods.cs

@@ -66,6 +66,7 @@ namespace OpenRA
 			// Several types of support directory types are available, depending on
 			// how the player has installed and launched the game.
 			// Read registration metadata from all of them
+			var stringPool = new HashSet<string>(); // Reuse common strings in YAML
 			foreach (var source in GetSupportDirs(ModRegistration.User | ModRegistration.System))
 			{
 				var metadataPath = Path.Combine(source, "ModMetadata");
@@ -76,7 +77,7 @@ namespace OpenRA
 				{
 					try
 					{
-						var yaml = MiniYaml.FromStream(File.OpenRead(path), path).First().Value;
+						var yaml = MiniYaml.FromFile(path, stringPool: stringPool).First().Value;
 						LoadMod(yaml, path);
 					}
 					catch (Exception e)
@@ -205,7 +206,7 @@ namespace OpenRA
 					string modKey = null;
 					try
 					{
-						var yaml = MiniYaml.FromStream(File.OpenRead(path), path).First().Value;
+						var yaml = MiniYaml.FromFile(path).First().Value;
 						var m = FieldLoader.Load<ExternalMod>(yaml);
 						modKey = ExternalMod.MakeKey(m);