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.
In the interest of the vast majority of naive users (who will never think about multiple registries), I'd like to see the package doc (which is the first thing a new user will read) starting at the convenience functions, and only then mention that there is a Registry interface with the same functions an methods.