Make deprecations "collectable"

I.e., make it possible to say, iterate over all deprecations that are defined.

Possibly this involves making `Deprecator`s take say, package names, such that "standard" use becomes:

```
regret = Deprecator(name=__package__)
```

and then each package can have its own deprecations iterated over on its corresponding deprecator instance.

Needs more thought though, and very likely relates/blocks #1.