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/Network/Handshake.cs

@@ -20,10 +20,10 @@ namespace OpenRA.Network
 		public string Version;
 		public string AuthToken;
 
-		public static HandshakeRequest Deserialize(string data)
+		public static HandshakeRequest Deserialize(string data, string name)
 		{
 			var handshake = new HandshakeRequest();
-			FieldLoader.Load(handshake, MiniYaml.FromString(data).First().Value);
+			FieldLoader.Load(handshake, MiniYaml.FromString(data, name).First().Value);
 			return handshake;
 		}
 
@@ -51,14 +51,14 @@ namespace OpenRA.Network
 		[FieldLoader.Ignore]
 		public Session.Client Client;
 
-		public static HandshakeResponse Deserialize(string data)
+		public static HandshakeResponse Deserialize(string data, string name)
 		{
 			var handshake = new HandshakeResponse
 			{
 				Client = new Session.Client()
 			};
 
-			var ys = MiniYaml.FromString(data);
+			var ys = MiniYaml.FromString(data, name);
 			foreach (var y in ys)
 			{
 				switch (y.Key)