Refactor symbol implementation to use UTF-8 bytevectors for names.

Refactor symbol implementation to use UTF-8 bytevectors for names.

Primitives handle this correctly since they accept either a legacy string or a
bytevector.  As long as no one peeks behind the abstraction this should be
transparent.

However, symbols with non-ASCII names will produce non-legacy strings when
asked.  AFAIK there are none currently in use.