Add "debug" stdlib

[moul]

Add a new standard library, "debug," to enhance {non,low}-interactive debugging.

Developers can import debug for tools like debug.PrintStack() and debug.Breakpoint() during local development. Importantly, when used on-chain, debug will have no effect. This means it's safe to include debug.XXX commands in published contracts or create a p/debugutil module.

Related with #999, #1226

cc @thehowl @mvertes