Avoid string allocations in MiniYaml parsing.

- Stream lines in as memory rather than needing to realise a string for each line, via a new method in StreamExts.
- Use span to avoid string allocations during parsing until we want to realise the node itself, in MiniYaml.FromLines.
- Change several callsites to use the streaming extension method rather than string method where possible.

OpenRA.Mods.Common/Widgets/Logic/ServerListLogic.cs

@@ -346,13 +346,13 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 				var games = new List<GameServer>();
 				var client = HttpClientFactory.Create();
 				var httpResponseMessage = await client.GetAsync(queryURL);
-				var result = await httpResponseMessage.Content.ReadAsStringAsync();
+				var result = await httpResponseMessage.Content.ReadAsStreamAsync();
 
 				activeQuery = true;
 
 				try
 				{
-					var yaml = MiniYaml.FromString(result);
+					var yaml = MiniYaml.FromStream(result);
 					foreach (var node in yaml)
 					{
 						try