Better-factored codegen

* src/pf/codegen.lua: Start of better access to packets.
Simplify parse output

* src/pf/parse.lua: Simplify to specify type in first element of table.
* src/pf/codegen.lua: Adapt.
Fix scoping bug in parser

* src/pf/parse.lua: Fix scoping bug.
Add pf.expand module

* src/pf/expand.lua: New file.
* src/Makefile: Add new file.