Improve printing of Orig RdrNames
In Tempate Haskell -ddump-splices, the "after" expression is populated with RdrNames, many of which are Orig things. We used to print these fully-qualified, but that's a bit heavy. This patch refactors the code a bit so that the same print-unqualified mechanism we use for Names also works for RdrNames. Lots of comments too, because it took me a while to figure out how it all worked again.