sha3, rng cleanup

[ounsworth]

Break up of #87, so this is actually dgh's submission. So I will be reviewing / approving.

Mechanical cleanups, no behaviour change.

use core:: imports, copy Copy types, drop a redundant zeroize

• std::marker::PhantomData -> core:: in sha3.rs, and std::fmt -> core:: in hash_drbg80090a.rs. Neither needs std; both crates are working towards no_std.
• KeyType and SecurityStrength are Copy, so min()/max() results are dereferenced rather than .clone()d. This also lets the surrounding expressions be formatted as single calls.
• Hash::hash_out() and XOF::hash_xof_out() called output.fill(0) before delegating to a path that zeroizes the buffer itself (do_final_out() and squeeze_out() respectively, per the trait contract). Dropped the outer fill; hash_out() no longer needs mut output or a reborrow.