feat: Stack Machine (rebased, starting on kernel compilation)

[acl-cqc]

Version of #88 rebased to come after #98, with a few commits moved out into #100

I've reworked the Hugr compiler to compile only Kernel boxes, and Compiler.hs (brat -c) calls this only for Kernel boxes that are inputs to the Id nodes of top-level decls. "compile" produces a Hugr + list of splices, where each splice is (the NodeId of a Hole op in the hugr, OutPort from BratGraph) - the "index" of the Hole op is not really used. The interpreter then (hopefully) evaluates the outports, perform the splices using #98, and returns the final Hugr in JSON form.

TODO:

• Add some actual interpreter tests
• Of course more interpreter stuff, nodes, wiring etc....could be a never-ending stream of later PRs tho
• Validation fails with a bunch of the output hugr .json files that are 0 bytes long?!?!