TODO update.

cranelift/isle/TODO

@@ -2,9 +2,18 @@
 
 - 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.
+
+- Support extern constants.
+
 - 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);`)
+  - 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.