Add a right-biased variant of 'NubList'.
GhcOptions: use `NubListR` where appropriate.

Some Cabal API clients apparently use `foldl mappend` on large lists of
`GhcOptions` records. Fixes #2110.
80-col violations.
Merge pull request #2111 from 23Skidoo/ghc-options-union

GhcOptions: use `NubListR` where appropriate.