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.Game/Map/MapCache.cs

@@ -183,9 +183,9 @@ namespace OpenRA
 					try
 					{
 						var httpResponseMessage = await client.GetAsync(url);
-						var result = await httpResponseMessage.Content.ReadAsStringAsync();
+						var result = await httpResponseMessage.Content.ReadAsStreamAsync();
 
-						var yaml = MiniYaml.FromString(result);
+						var yaml = MiniYaml.FromStream(result);
 						foreach (var kv in yaml)
 							previews[kv.Key].UpdateRemoteSearch(MapStatus.DownloadAvailable, kv.Value, mapDetailsReceived);