Create Foreign Keccak256 Function

[Savio-Sou]

Problem

We already have the Keccak256 opcode in ACVM, but we're missing a Noir function to actually call it in Noir programs.

Proposed solution

Create the function in stdlib.

Additional context

From @vezenovm on Slack re how the function signature could look like:

I think there were some efficiency benefits to using a [u64] however a byte slice is more intuitive and perhaps we can add helper methods to convert between different array types

Submission Checklist

• Once I hit submit, I will assign this issue to the Project Board with the appropriate tags.

[colinnielsen]

Looking forward to this. I wrote an ECDSA recover-like circuit, using @TomAFrench's keccak function signature.
I had to manually convert my inputs to [u64;16], then pass the input bits.

It would be amazing to have a circuit signature of keccak([u64]) or keccak([u8]) that handled the input_length in bits automatically