Add :gen/return support in malli.generator

[radhikalism]

Resolves #932.

Adds idiom for :gen/return x in properties, just like clojure.test.check.generators/return x. It has similar effect to more indirect workarounds previously like :gen/elements [x] or :gen/gen (gen/return x) or :gen/fmap (constantly x).

Combining :gen/return and :gen/fmap works similarly to combining :gen/return and :gen/elements. Returning nil from :gen/return nil is supported.

[ikitommi]

Piilomaan Pikkuaasi 👍

[ikitommi]

Thanks!