Use spans to improve performance in StreamExts.

Also avoid ReadBytes calls that allocate a buffer by either updating the stream position (if not interested in the bytes), by reusing an input buffer (if interested in the bytes), or using a stackalloc buffer to avoid the allocation (for small reads).

OpenRA.Mods.Common/FileSystem/InstallShieldPackage.cs

@@ -73,7 +73,7 @@ namespace OpenRA.Mods.Common.FileSystem
 						var dirName = s.ReadASCII(nameLength);
 
 						// Skip to the end of the chunk
-						s.ReadBytes(chunkSize - nameLength - 6);
+						s.Position += chunkSize - nameLength - 6;
 						directories.Add(dirName, fileCount);
 					}