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.
As a minimal change, you could just write:

To expose metrics registered with the Prometheus default registry, an HTTP server needs to know about the Prometheus handler. The usual endpoint is "/metrics".

A cleaner approach would probably be to start the package doc comment with a short explanation that directs power users to the Registry interface and explains that this introduction is mainly concerned with the top-level convenience functions that refer to the default registry.

I'd like to make sure that the package doc-comment enables naive first time users to set up a couple of metrics and go.