Implement a string to byte-array conversion method

[iAmMichaelConnor]

Problem

Currently, strings can't used in many useful ways within the language. The ability to convert a string to a [u8; N] might help.

An example issue we're looking at is this: AztecProtocol/aztec-packages#885

Happy Case

let string = "hello";
let be_bytes = string.to_be_bytes();
let le_bytes = string.to_le_bytes();

(And maybe even the ability to convert back to a string would be nice?)