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.