Move backend tests to check target

* src/Makefile (check): Move backend tests here.
Start of new codegen pass

* src/pf/backend.lua: Add lowering to one-armed ifs and labelled blocks.
Add low-level simplifier

* src/pf/backend.lua: Add a simplification over low-level code.
Simplify labels.

* src/pf/backend.lua: Add label simplification.