Editorial: Refactor algorithm conventions (#919)

* Pull in algorithm conventions from 6.2.3 around implicit completion
  values and the like
* Define syntax-directed operations
* Define runtime semantics
* Define how production associated algorithms are invoked
* Clarify that all algorithms for runtime semantics return completion
  records (including production-associated algorithms).
* Specify that production-associated algorithms may make use of ?/!
  shorthands