v0.0.0-alpha.5: Add STM-based Anchor implementation. (#29)

* Implement software-transactional memory (STM) in package stm.

* Implement basic transactional API for STM-based Anchor trees.

* Implement generic bounded types.

* Implement bounded Path type & update cluster.capnp to use path strings.

* Add Path indexer.

* Test binding path from parts & children

* Rename anchor.Anchor to anchor.Scheduler & test initialization.

* Implement STM-backed root anchor.

* Refactor cluster and anchor capability schemas.

* Remove orphaned anchors.

* Major refactor.  Bundle anchor & view in host cap.

* Add TODO for LoadValue()

* Add TODO for client.Node