We're maybe not exporting `let*`, `let+` and `and+`

[mlms13]

Back in the day, we had added support for OCaml's binding-ops for everything that implemented monad and applicative/functor. I think maybe that work never got merged because Bucklescript refused to support this feature?

Now that we're on Melange, we should definitely add those back. And also, we should add a special case let (let*) = bind to modules like IO and Result that don't technically implement Monad (because they're t('a, 'e) instead of t('a)).