Final comment period for Debug Builders stabilization

@alexcrichton Making it harder to screw up is a decent reason to prefer self to &mut self, IMO. If we went with a &mut self model, what would we want the behavior of an entry/field call after finish to be? Options I see are panicking, returning a fmt::Error or going on and printing anyway.

@seanmonstar Yep, something like that seems reasonable, though I’d guess we’d add it unstable for a bit to make sure we were happy with it. In particular, I’m trying to make sure to craft these APIs to minimize binary size (this is why the methods take &Debug trait objects instead of monomorphizing for example). Probably something like

impl<'a, 'b> DebugMap<'a, 'b> {
    fn entries<K, V, I>(self, it: I) -> DebugMap<'a, 'b>
        where K: Debug, V: Debug, I: IntoIterator<Item=(K, V)> { .. }
}