Depend on binary 0.7 on >= GHC 7.8

While this isn’t needed (GHC 7.8 ships with binary 0.7, we indirectly
depend on GHC), it’s more correct and might just prevent future
headache.
Use Proxy in Yi.MiniBuffer
Define action* in Yi.Lexer.Alex using lambdas
Remove pureM from Yi.Core
Add docs to combinators in Yi.Keymap.Keys
Use Mode Binary instance
Use a <$> instead of pure a <*>
Use OverloadedStrings and IsString with Data.Rope
Use lenses a bit more in Yi.Buffer.Misc
Use lens operators more in Yi.Buffer.Misc
Use convenience operators more and add docs in a few places