High level flows in Courgette has messy error-handling code that's
hard to read. This CL adds CourgetteFlow to absorb commonly used data
and error-handling code, and improve readability of high-level logic.
Additional changes:
- Fix Courgette-disadj (debugging flow), which likely was broken by
  http://crrev.com/2793153003; Adjust() is called but AssemblyProgram
  did not annotate Labels.
- Add BasicBuffer interface, implemented by RegionBuffer and (existing)
  BufferedFileReader.
- PatchGeneratorX86_32::Transform(): Implemented TODO to instantiate
  "new" AssemblyProgram to reduce Courgette-gen peak memory.
- courgette_tools.cc: Make help text more uniform.

BUG=660980

Review-Url: https://codereview.chromium.org/2827103002
Cr-Commit-Position: refs/heads/master@{#467157}