Enforce Compute limits, optionally

[acw]

Compute enforces a few limits on code executing, which are only partially replicated in Viceroy. It would be nice if we could replicate these limits -- ideally in an automated way, so if the Compute limit changes the Viceroy one does, too -- that would be great.

See this document for more information: https://docs.fastly.com/products/network-services-resource-limits

If/when we do this, we should also support a flag that disables these checks and just emits warnings, to support a general prototyping workflow.