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/LocalPlayerProfile.cs

@@ -83,9 +83,9 @@ namespace OpenRA
 					var client = HttpClientFactory.Create();
 
 					var httpResponseMessage = await client.GetAsync(playerDatabase.Profile + Fingerprint);
-					var result = await httpResponseMessage.Content.ReadAsStringAsync();
+					var result = await httpResponseMessage.Content.ReadAsStreamAsync();
 
-					var yaml = MiniYaml.FromString(result).First();
+					var yaml = MiniYaml.FromStream(result).First();
 					if (yaml.Key == "Player")
 					{
 						innerData = FieldLoader.Load<PlayerProfile>(yaml.Value);