Consider if other order of combination is more appropriate for the monoids on config types

Many of the fields use `configField l <|> configField r`. This means the value on the left overrides the value on the right. I noticed that this isn't uniformly followed. For example,

> ```
> , configMonoidGhcOptions = Map.unionWith (++) (configMonoidGhcOptions l) (configMonoidGhcOptions r)
> ```

I would expect the order to be the opposite here, since ghc-options later in the list override earlier options. These definitions should be considered more carefully.

For now, in working on https://github.com/commercialhaskell/stack/issues/863, I am sticking with the same semantics as before. So, if this gets changed, that should also get modified.