chore: add common crate for project wide definitions

[atanmarko]

Resolves #261

[0xaatif]

I'd love some tests to show the semantic meaning of each of these constants,
and some better documentation
"we expect these cases to be common enough that it's worth inlining".

Though I should state my preference that we don't hard-code anything anyway and eat any perf impact as I suspect it's negligible

#[test]
fn test() {
    assert_eq!(EMPTY_CODE_HASH, keccak_hash::keccak([]));
}

[BGluth]

Yeah this was needed and something I was thinking about for a bit. Thanks for doing this!