Function group 

pub fn group(
    func: &mut Func,
    names: &mut Interner,
    conv: &CallRegs,
    counting: bool,
) -> Ran

Runs the whole group over one function, in the order Step::GROUP gives.

This is the entry point section 36.1 asks for. Every caller wanting a function lowered calls this and nothing else, so adding a lowering is adding it to Step::GROUP rather than to whichever line of crate::pipeline looked convenient.

counting is whether to work out what each step found and left, which is what Lowerings::wanted answers and which costs what it says there. The steps run either way and the function comes out the same; what a false gives back is an empty Ran.