Add a Control construct and re-engineer closure conversion

[Hugobros3]

This 🚧 draft 🚧 PR adds a control construct as found in Shady and performs a significant amount of refactoring on the IR to support closure conversion more robustly. This is not currently merge-able and exists as a tracking issue.

Current changelist:

• Add control intrinsic and matching JoinPointType
• Add an explicit ReturnType type that encodes the codomain of a function.
  • Use that as the ground-truth for whether a function is returning or not, in place of the order
• Refactored a bunch of passes to use a unified Rewriter
• Improve scoping analysis to produce a nesting
  • Made the scheduler::early use the new scoping analysis
  • Add a new World::scoped_dump that uses the new scope analysis to pretty-print IR
  • Add a graphviz IR dumping utility, that can also dump the world with scoping
• Add a lift2cff pass
• Rewrite closure conversion entirely and make it robust

The lowering for this is tricky: setjmp/longjmp cannot be safely used inside functions because they can overwrite side effects that have happened in-between the control call and consuming the token. One solution is to emit cps-style code instead of direct-style, and then we can use tailcalls and perform manual stack pointer adjustments instead.

However there are backends where this will not be permissible (those who lack true function calls mostly) and we will need to find an alternative solution for those.

Still TODO:

• 🚧 Per-backend passes (in a way that doesn't break the kernels analysis)
• Lower control to something sensible in non-shady backends
• Aggressively eliminate closures for call-challenged backends