Large rewrite/refactoring of the spec (#156)

This fixes a number of pending issues listed below,
for which there was consensus, but editing was still pending.

This introduces a dependency on the infra standard [[INFRA]],
to define precisely the low-level data structures
used by slots and concepts.

This also specifies slots more clearly.

Additionally, this rewrite sprinkles inline issues
in areas of the specs which require more work.

* Handle GC issues by getting rid of the SensorReading interface. 
  Closes #153. Closes #101.
* Check sensor existence before requesting permission. Closes #145.
* Remove side effects from constructors. Closes #138.
* Move default sensor check to start() and add "unconnected" state.
  Closes #128 and closes #104.
* Throttle update frequency to 60Hz by relying on
  requestAnimationFrame. Closes #100.