Use ::core in lieu of ::std, allowing this crate to be usable in #![no_std]
environments.
Gates features that presently depend on ::std (presently just rand) behind a
"std" cargo feature, which is enabled by default.
This adds a dependency on the `rand` crate, used to construct an
OS-backed CSPRNG. The implementation in this commit is somewhat
inefficient as it constructs a new OsRng object every time; it might be
better to construct it once. (Seems like a lot of overhead for a few
getrandom(2) calls...)
