Support hot reloading and other long-lived dynamic build lifecycles

[cassaundra]

This involves:

Core tasks:

• Adding concurrent builds with interruptible handles (Support concurrent tasks #15)
• Adding a file watcher mode which responds to changes and rebuilds as necessary (Add file watching mode #14)

Auxiliary issues:

• Ensure non-concurrent tasks get run on the main thread (Non-concurrent tasks don't get run on the main thread #24)