Subclass unicode for String and Symbol

Refactoring the String and Symbol classes into subclasses of unicode decouples
from SExpBase. Neither of these classes will creep features. The behavior of
__eq__ has been preserved and doctests were added. Two __sexp__ methods were
used for consistency---registering directly to tosexp() might be faster.
Petrified Quoted and Delimiters, obsoleted SExpBase
more exception coverage