Move transaction script into a separate crate

From https://github.com/0xMiden/miden-faucet/pull/56#pullrequestreview-3171361734:
> In the longer run, we should put the transaction script MASM into a separate crate and make it appear as a "package". This is how eventually the compiler will be used to create transaction scripts in Rust (i.e., there will be a separate crate per transaction script).
>
> The same crate may contain some kind of "constructor" that could make building transactions using this script easier.