Update rand

[JulianKniephoff]

@LukasKalbertodt did this in 543535f, but I reverted it since (see 71aad47), because I didn't like how it forced me to specify the transitive dependency to getrandom (and its version) explicitly, only to add the wasm-bindgen feature there.

I hope we can circumvent that at some point, and maybe the warning mentioned in #4 will be gone then, as well.