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.
Switches from the "rust-crypto" crate (which is not #![no_std] compatible) to
the sha2 crate, which is factored out of the original "rust-crypto" project
and being actively maintained.
When using the 'licence-file' directive, crates.io states that the license is
"non-standard", when really it should say that the code is CC0 (public
domain).