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/FileFormats/IniFile.cs

@@ -35,13 +35,9 @@ namespace OpenRA.Mods.Common.FileFormats
 
 		public void Load(Stream s)
 		{
-			var reader = new StreamReader(s);
 			IniSection currentSection = null;
-
-			while (!reader.EndOfStream)
+			foreach (var line in s.ReadAllLines())
 			{
-				var line = reader.ReadLine();
-
 				if (line.Length == 0) continue;
 
 				switch (line[0])