Gardening

This was to get me motivated for #168 ;) Again, WIP pending the aforementioned PR.

- Don't define module_functions for HeaderNegotiation
  - Makes render_error() consistent with other places which call the ensure_* helpers.
- Shrink Trace::FSM's surface
  - Relieve Decision::FSM from the knowledge of Trace::FSM's API, and shrink the surface to only #tracing_enabled?.