Helpers for ensuring that values are wrapped in a certain type

The counterparts of fns like `from-maybe`, take a value and ensure that they are wrapped in the correct type.

An example of how it'd behave for Maybe:

``` clojure
(require '[cats.monad.maybe :as maybe])

(maybe/to-maybe (maybe/just 42))
;; => #

(maybe/to-maybe 3)
;; => #

(maybe/to-maybe nil)
;; => #

(maybe/to-maybe (maybe/nothing))
;; => #
```