WIP: Deterministic and varied pure/impure fn gen

[frenchy64]

Close #1039

FWIW I haven't worked out how to make functions shrinkable.

The implementation can be further simplified by using the private gen/randomized function instead of seeded. That might fix the babashka tests.

;; This sequence of results should be reproducible.
(def pure   (mg/generate [:=>                    [:cat :int] :int] {:seed 0 :size 10}))
(def impure (mg/generate [:=> {:gen/impure true} [:cat :int] :int] {:seed 0 :size 10}))
(repeatedly 10 #(pure 2))
;=> (-106 -106 -106 -106 -106 -106 -106 -106 -106 -106)
(repeatedly 10 #(pure 2))
;=> (-106 -106 -106 -106 -106 -106 -106 -106 -106 -106)
(repeatedly 10 #(impure 2))
;=> (0 -1 0 -3 0 1 16 0 7 3)
(repeatedly 10 #(impure 2))
;=> (0 -1 -1 1 2 3 -2 38 -5 -12)

(mapv pure (range 10))
;=> [5 511 -106 20 -51 -322 1 0 434 -1]
(mapv pure (range 10))
;=> [5 511 -106 20 -51 -322 1 0 434 -1]
(mapv impure (range 10))
;=> [-1 0 -2 -4 -3 -4 -15 -15 -16 -2]
(mapv impure (range 10))
;=> [-1 -1 -1 0 1 3 -12 30 -2 1]