Add some new row instances and remove some dead code.

[iand675]

Adds few trivial instances for base's functors like Identity and Proxy. Also removes CPP conditionals for base versions < 4.6 since they're no longer supported anyways (according to the cabal file).

[BardurArantsson]

If a "cleaner" commit log is desired, I think I've got all the "remove support for old GHC" bits in my unmerged PR #228 commits. (Just a suggestion, I only care about the removal of obsolete code :) )