Simplify codegen to work on expanded expressions

* src/pf/codegen.lua: Update to work on expanded expressions.
Better conditional folding

* src/pf/expand.lua: Rotate asserts out of test position to expose test
  expressions to conditional folding.
New compiler has length argument

* src/pf/codegen.lua: Add length argument to predicates.
* src/pf/expand.lua: Print less.
Remove prints

* src/pf/codegen.lua:
* src/pf/expand.lua: Remove more prints.