Add unapprove instruction

[archseer]

Reverse of approve (I just copied approve and flipped the bool), allows a signer to undo a previous approval.

[fanatid]

Maybe will better to change approve for accepting bool as second argument?

pub fn approve(ctx: Context<Approve>, state: bool) -> Result<()> {
    ctx.accounts.transaction.signers[owner_index] = state;

[archseer]

We could but it would be a breaking change to the current API

[fanatid]

If we want to skip breaking change we can introduce approve2 like initialize_account2 in spl-token.

[archseer]

I can implement it either way, let me know if you'd like me to change it.

[fanatid]

ping @armaniferrante