An assortment of small changes.

[reem]

See commit messages for the changes made.

Sample of new benchmark output:

   Compiling tiny-keccak v1.0.5 (file:///Users/reem/code/rust/tiny-keccak)
     Running target/release/sha3-149179c4c09e667e

running 2 tests
test bench_sha3_256 ... bench:      19,664 ns/iter (+/- 1,589) = 208 MB/s
test keccakf_u64    ... bench:         616 ns/iter (+/- 30) = 40 MB/s

test result: ok. 0 passed; 0 failed; 0 ignored; 2 measured

     Running target/release/tiny_keccak-40bcaa9811480135

running 7 tests
test tests::empty_keccak ... ignored
test tests::empty_sha3_256 ... ignored
test tests::long_string_sha3_512 ... ignored
test tests::long_string_sha3_512_parts ... ignored
test tests::string_sha3_256 ... ignored
test tests::string_sha3_256_parts ... ignored
test tests::string_sha3_256_parts5 ... ignored

test result: ok. 0 passed; 0 failed; 7 ignored; 0 measured

Note the MB/s annotations.

[debris]

Thanks for contribution! :+1: