Bugfix release.

• Bugfix for select_first() on an rvalue map entry.

Document all the things.

• Add loads of documentation. (Still needs more.)
• Various bug fixes.
• Add push() to async adapter.

Initial release

Objpipe was developed as part of a program, but it's become too nice to keep it there.

So I moved it into its own library.

Note: the interface is still a bit in flux.
While most of the end-user functionality is pretty stable, the internals may change a bit.
In particular, I want to split off the implementation details in such a way that people can easily create their own transformations and sources.