Fix vary function (#268)
Fix vary function (#268)

* Switch to a prefix-free code for variant

This commit fixes a bug where `vary(k, rng)` always returns
`rng.split.0` which in turn led to `Gen.variant(seed)` ignoring the `seed`
argument

See
https://github.com/nick8325/quickcheck/commit/71477ceb62c68cdceaf2c3454a0c10d0e55b0326
for the corresponding change in QuickCheck
typelift/SwiftCheck,CodaFi/SwiftCheck,CodaFi/SwiftCheck,CodaFi/SwiftCheck,typelift/SwiftCheck,typelift/SwiftCheck,CodaFi/SwiftCheck,typelift/SwiftCheck,typelift/SwiftCheck,CodaFi/SwiftCheck