Generic snapshot-and-run driver for all runtimes (pyhl for Node, etc.)

[danbugs]

Summary

pyhl provides a great UX for Python: `setup` warms the interpreter + preloaded packages, snapshots, and `run` restores hermetically at ~100ms. We need the same for other runtimes — Node.js with npm packages, Go, .NET, etc.

Design questions

• Can we generalize `pyhl` into a single `hlrun` (or similar) binary parameterized by runtime?
• How does package management vary? Python has pip + a flat site-packages; Node has npm + node_modules; Go compiles statically; .NET has NuGet
• What's the warmup sequence for each runtime? (Node: require preloads; .NET: assembly load; Go: n/a — static binary)
• Should each runtime have its own binary (nodehl, dotnethl) or one generic driver with runtime configs?
• How do we handle the CPIO build step generically?