add INLINEABLE to maybe, either, bool

Its come to my attention that maybe, either, and its new sibling bool, all lack the INLINEABLE attribute, or its more aggressive sibling INLINE this seems like one of those operations where inlining in client use sites is a good option to have, and currently not possible! theres probably other stuff that would benefit from an INLINEABLE pragma in base, but this is an obvious, simple, "easy win" that I noticed when Oliver's patch got merged into base. Thoughts? Time scale: sometime this week? (ghc 7.8 merge window is landing!) cheers