fix some mixed space+tab indentation

This fixes all instances of " \t" in the code base. Most common case
seems to be after a "where" line; probably vim copied the two space layout
of that line.

Done as a background task while listening to episode 2 of the Type Theory
podcast.

Types/MetaData.hs

@@ -290,4 +290,4 @@ prop_metadata_serialize f v m = and
 	, deserialize (serialize m') == Just m'
 	]
   where
-  	m' = removeEmptyFields m
+	m' = removeEmptyFields m