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?.
Merge pull request #171 from seancribbs/gardening

Gardening
Make error HTML more console-friendly
Add HTTPkit adapter
Deprecate Hatetepe adapter