- Document the semantics of the DSL!

- Clean up and factor the codegen properly.

- Get rid of the expression syntax `<EXPR` in patterns; do a type-dependent
  parse instead where we know the polarity of pattern-term args and parse
  in-args as exprs.

- Look into whether optimizations are possible:
  - More in-depth fallibility analysis (avoid failure edges where possible)

- Slightly nicer human-readable generated code
  - Include full rule body (S-expression) in comment, not just line number
  - Inline some expressions (no more `let val23 = 1234; ... f(val23);`)

- Build inlining and simplification: inline invocations of internal
  constructors, and eliminate ctor-etor or makevariant-matchvariant pairs.

- Ideas from discussion with fitzgen
  - Turn arg-polarity and exprs on extractors into purer "InstFormat"
  - Emit two contexts: an immutable context for inputs and a mutable context for
    outputs