(prop) Canonical property storage

Provide a means by which the string key/value pairs in a Properties object can be stored in a canonical form that is independent of time or any other host-, system-, or implementation-dependent detail.

This could be as simple as a Properties::storeCanonical method that would write the entries in key-name order, preceded by an optional user-supplied comment, but not including a comment with the date.

This would be useful for, among other things, reproducible builds.