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/UpdateRules/Rules/20210321/RemoveRenderSpritesScale.cs

@@ -23,7 +23,7 @@ namespace OpenRA.Mods.Common.UpdateRules.Rules
 		{
 			foreach (var renderSprites in actorNode.ChildrenMatching("RenderSprites"))
 				if (renderSprites.RemoveNodes("Scale") > 0)
-					yield return $"The actor-level scaling has been removed from {actorNode.Key} ({actorNode.Location.Filename}).\n" +
+					yield return $"The actor-level scaling has been removed from {actorNode.Key} ({actorNode.Location.Name}).\n" +
 						"You must manually define Scale on its sequences instead.";
 		}
 	}