rand: remove rand as a public dependency

This removes the use of the rand_core crate as a public dependency. It
is now an implementation detail.

We achieve this primarily by turning the `Gen` trait into a concrete
type and fixing the fallout.

This does make it impossible for callers to use their own `Gen`
implementations, but it's unclear how often this was being used (if at
all). This does also limit the number of RNG utility routines that
callers have easy access to. However, it should be possible to use
rand's `SeedableRng::from_{rng,seed}` routines to get access to more
general RNGs.

Closes #241

Cargo.toml

@@ -28,5 +28,5 @@ name = "quickcheck"
 [dependencies]
 env_logger = { version = "0.7.0", default-features = false, optional = true }
 log = { version = "0.4", optional = true }
-rand = "0.7"
+rand = { version = "0.7", features = ["small_rng"] }
 rand_core = "0.5"