Add "port" expander.

* src/pf/expand.lua: Add expander for "port".
Simplify IR

* src/pf/expand.lua:
* src/pf/codegen.lua: Change "constant" to "true"/"false", and transform
  "and" and "or" to "if".
Fixes

* src/pf/expand.lua: Tweak debug printing.
* src/pf/codegen.lua: Fix true/false/fail.
Better if code generation

* src/pf/codegen.lua: Better code generation for if.