inter-target ordering

[pufferfish101007]

very very WIP, nowhere near ready. creating a PR to make a note of what needs to be done.

will resolve #59

Changes

• driveby: represent WASM types as rust types 👀
• replace global threads table with an array of arrays of threads, with one array per target

Still TODO

• Finish implementing mutually recursive type registration
  • need a new IntraRecGroupInfo or similar context in which to register RecGroupDependencies and co.
  • add new CompoundType for a rec group item so that rec group registrations have a canonical representation and can be registered uniquely
  • handle transformation from rec group CompoundType to actual WASM type - should we encapsulate into a real rec group or not? unclear if that's necessary
• adapt remaining instructions & functions to use new thread layout