Add keccak256 hashing algorithm

[gusinacio]

Javascript and Rust are widely used on the Ethereum ecosystem and I can see a future where they can use Gleam a simpler language than Rust with static typing to replace Javascript.

I suggest adding keccak256 to enable libraries to use the standard gleam/crypto lib.

[lpil]

Do the Erlang and JavaScript standard libraries have this algorithm? If not then we cannot add it here, so it would need to be in a different package.