Remove packet abstraction.

- Inline operations for reading words (u8, u16, s32) in buffer.
- Inilining reduced generated asm code considerably.
Merge pull request #8 from dpino/remove-packet-abstraction

Remove packet abstraction.
Minor codegen cleanup

* src/pf/codegen.lua: Minor cleanup.
More codegen-time CFG simplifications

* src/pf/codegen.lua: More minor CFG simplifications.
More conditional simplification

* src/pf/expand.lua: Rotate duplicate tests out of if expressions.