Derive MonadBase instance for InputT

[mitchellwrosen]

This change is 

[judah]

Thanks for the patch.

I'm generally conservative about these sorts of changes because haskeline is a dependency of ghc, so any dependencies of haskeline become dependencies of ghc. See also a similar discussion in #24 about making haskeline depend on the "exceptions" package. If you can get consensus from the ghc-devs about depending on making such a package a boot requirement, then I'd be fine with adding the dependency.

In the meantime, maybe it's worth making an intermediate package with the orphan instance? Is that feasible using Control.Monad.Base.liftBaseDefault?

[mitchellwrosen]

Ah, I didn't realize this is a dependency of GHC! For now, I'll just add an orphan instance in my app.