fix Layered's max_level_hint impl

[guswynn]

Motivation

Fixes: #2265

Solution

When there are no per-layer filters, the Layered pick_level_hint function is wrong. It prioritizes Some(_) over None, even though None is basically Some(TRACE). This was not notices because most people are doing one of:

1. global filtering which works differently (as far as I can tell)
2. per-layer filtering, whose code-path seems to work (it also short-circuits None)
3. 3. the fmt subscriber, which happened to work with this change

In this branch we also have to fix the fmt Collector, which implements Collect and erroneously passes through to a layer that unconditionally returns None. This is the correct option for the layer itself, but overrides the behavior we want for the subscriber, which is to follow the F field directly. We set inner_is_registry directly to combat this problem

[guswynn]

closing in favor of #2321