Semantics of using reserved encodings; constraints on unspecified behavior

[aswaterman]

This is a placeholder for the issue discussed in #584, and for the broader topic of which behaviors are allowed when reserved encodings are used.

There is an interaction with the Ssstrict extension, but most of the interesting questions arise when Ssstrict is not in effect, when using a reserved encoding becomes unspecified. How to constrain that unspecified behavior is an ongoing topic of discussion.