RFC: centralise registries

[Ian Clatworthy]

Robert Collins wrote:

> I don't think potential extraction of code is a good reason to keep our
> instances and our support classes separate; putting a bunch of instances
> in registry.py doesn't make it harder to pull out the class definition
> to a new project.

It's a layering issue: lower layers shouldn't know about higher layers
any more than they need to.

Ian C.