Function shorter 

pub fn shorter(
    func: &mut Func,
    short: &ShortInsts,
    flags: &FlagInsts,
    machine: &MachineInsts,
    names: &mut Interner,
    goal: Goal,
) -> usize

Rewrites every instruction that has a shorter spelling nothing would notice.

Gives back how many were rewritten, which the tests read and nothing else does.