Add closure parsing to at-spawn

[jpsamaroo]

This PR brings Dagger.@spawn closer to Threads.@spawn by allowing an alternative parsing mode when begin ... end blocks are encountered; specifically, such blocks are treated as a function without any arguments, which closes over any captured values.

Todo:

• Detect closed-over variables (if possible) and pass them as arguments, in case they're thunks/chunks
• (Alternative) Recurse into closure at runtime to find thunks/chunks and replace them during move
• (Optional) Implement recursive parsing mode
• Add tests
• Add docs

Fixes #421