Add deferManyElse and deferEither to Heist.Compiled

[kaol]

I would like to propose these two new functions for Heist.Compiled.

deferManyElse is useful for showing "no results" or some such message when passed an empty Foldable. I was considering maybeDefer as the name since it's analogous to maybe but maybe is useful for just Maybe and not for Foldable in generic.

deferEither is much like either but for splices. At least my code's full of splices that do DB operations and I need to do something with all the runtime Lefts.

I'm not proposing adding any deferManyElseMap or deferEitherMap, since deferEitherMap f a b n can be written as just deferEither a b $ f =<< n. Same goes for deferManyElseMap.

If it was up to me, I would deprecate both deferMap and mayDeferMap, since they can be substituted with defer and deferMany, respectively.

[mightybyte]

This looks good, but it's not building on older GHCs (see https://travis-ci.org/github/snapframework/heist/builds/637120129). Any chance we could preserve that compatibility?

[kaol]

Easy, it just needs to use foldr (\_ _ -> False) True in there, which is the current definition of null. Old versions use pattern matching and the type is restricted to [a] -> Bool. Just a moment and I'll refresh this branch.

[mightybyte]

Hmmm, it doesn't look like CI was triggered. Perhaps that's because it was a force push?

[mightybyte]

I created another local branch and that doesn't seem to have fixed it. https://travis-ci.org/github/snapframework/heist/builds/701697177

[kaol]

I amended and force pushed another version of this. Copying null from new base wasn't enough but it needed a new foldr as well. I tested the definition at home with 7.8.4 and 8.4.4.