Upgrading to rand 0.8

[dmit]

A new version of the rand crate has been released. While the changes needed to upgrade time itself are trivial, quickcheck still depends on rand 0.7.3 and it is unclear when a new version will be released (although there's already a PR at BurntSushi/quickcheck#264).

It is technically possible to rely on both 0.7.3 (for quickcheck) and 0.8.0 (for everything else) by doing something like this:

[dependencies]
rand = { version = "0.8.0", optional = true, default-features = false }
rand-07 = { package = "rand", version = "0.7.3", optional = true, default-features = false }

[features]
quickcheck = ["quickcheck-dep", "rand-07", "std"]

Although it's not very pretty. I'll submit a PR to give a better idea of what the changes look like.

MSRV is not affected, since rand 0.8 supports Rust >=1.36.