Make Registry public

This change set exports a Registry interface implemented by the local
registry type. I added TODO comments along the way with improvements
that could be done to reduce the surface area of a Registry's behaviour
and to make the code more orthogonal overall. I'd be glad to contribute
these changes over time, if you like the direction I propose.

This has been done on top of the `standard-exports` branch.

All tests pass except for the Linux specifc `/proc` related tests. I
suggest that we add linux specific build tags to those files.
OK, that's kind of inconsistent with my remark below to only mention custom registries further down...

The implementation details don't matter so much. I just want to keep things as little surprising for naive users as possible. Starting with the simple stuff, dig down into power-user features further down...