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/Widgets/WidgetLoader.cs

@@ -25,7 +25,9 @@ namespace OpenRA
 		{
 			this.modData = modData;
 
-			foreach (var file in modData.Manifest.ChromeLayout.Select(a => MiniYaml.FromStream(modData.DefaultFileSystem.Open(a), a)))
+			var stringPool = new HashSet<string>(); // Reuse common strings in YAML
+			foreach (var file in modData.Manifest.ChromeLayout.Select(
+				a => MiniYaml.FromStream(modData.DefaultFileSystem.Open(a), a, stringPool: stringPool)))
 				foreach (var w in file)
 				{
 					var key = w.Key[(w.Key.IndexOf('@') + 1)..];