Move implementation of hash functions outside stdlib leaving just their permutation functions

[TomAFrench]

We've been working towards removing as much code as possible which isn't necessary to be in the stdlib as part of #1258. At the same time we've been replacing black box hash functions with their permutation functions.

This means that we have a fair number of very high risk code in the stdlib which would make it difficult to mitigate any potential vulnerabilities in future (moving people off vulnerable code would require updating to a whole new version of Noir).

We should move these out to live in external libraries while maintaining the same test coverage (e.g. fuzzing).

[TomAFrench]

As discussed in the call, we have fuzz testing for these functions in https://github.com/noir-lang/noir/blob/master/tooling/nargo_cli/tests/stdlib-props.rs.

Two potential options for how to handle this when moving the libraries out would be to maintain a test harness which will compile and execute the noir while checking against a reference implementation of the hash functions. The other option would be to use foreign calls and an RPC server which will return the output of a hash function.

#[test]
unconstrained fn test_keccak256(x: [u8; 50]) {
    let hash = keccak256(x, 50);

    assert_eq(external_keccak256(x, 50), hash);
}

#[oracle(external_keccak256)]
unconstrained fn external_keccak256(input: [u8; 50], len: u32) -> [u8; 32] {}

This just requires an RPC server which will respond with the necessary hash and it potentially easier for an external developer to replicate.