Add malli.generator support for :gen/return

[radhikalism]

When generating values from schemas, it is not possible to directly express returning a single constant value, as in clojure.test.check.generators/return.

Expected: (mg/generate [:int {:gen/return 1}]) => constantly 1

Actual: Other values are returned because :gen/return is not recognized.

Workarounds:

• (mg/generate [:int {:gen/gen (gen/return 1)}])
• (mg/generate [:int {:gen/elements [1]}])
• (mg/generate [:int {:gen/fmap (constantly 1)}])

Although workarounds are available, they are not as idiomatic as :gen/return x corresponding to (gen/return x).