Add some property tests

[tfausak]

let f = from @s @t
let g = from @t @s
let s1 = ...
let t1 = f s1
let s2 = g t1
-- s1 /= s2 in general, consider going between (List a) and (Set a)
let t2 = f s2
t2 == t1
let s3 = g t2
s3 == s2