Add ssa.lua

* src/pf/anf.lua: Separate the labels language from anf.lua.
* src/pf/ssa.lua: New file.
* src/pf/Makefile: Add to tests.

* src/pf/utils.lua: Move fixpoint here.
* src/pf/optimize.lua: Adapt caller.
Add codegen2 from SSA

* src/pf/codegen2.lua: New file.
* src/Makefile: Add to build.
New codegen module works

* src/pf/codegen2.lua: Update to fix syntax of generated code.
New backend passes tests

* src/pf.lua: Add "codegen2=true" block to compile_filter, and wire up
  tests.

* src/pf/codegen2.lua:
* src/pf/ssa.lua: Instead of adding "not" nodes, invert boolean
  expressions by changing the operator.
Better code generation

* src/pf/codegen2.lua: Splice the body of a "do" in tail position.
React to feedback, remove a print

* src/pf/codegen2.lua: Remove a print.
* src/pf/ssa.lua: Remove some unused variables and consistently name
  ipairs variables.
Update codegen2 for feedback

* src/pf/codegen2.lua: Cleanups from code review.
Merge pull request #92 from Igalia/wip-ssa

Wip ssa