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.Mods.Common/Widgets/Logic/MissionBrowserLogic.cs

@@ -128,8 +128,9 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 			// Add a group for each campaign
 			if (modData.Manifest.Missions.Length > 0)
 			{
+				var stringPool = new HashSet<string>(); // Reuse common strings in YAML
 				var yaml = MiniYaml.Merge(modData.Manifest.Missions.Select(
-					m => MiniYaml.FromStream(modData.DefaultFileSystem.Open(m), m)));
+					m => MiniYaml.FromStream(modData.DefaultFileSystem.Open(m), m, stringPool: stringPool)));
 
 				foreach (var kv in yaml)
 				{