Avoid stripping T.Comment tokens contained within an sql.Comment before
stripping the sql.Comment itself.  Now an sql.Comment token will be
stripped first along with any contained T.Comment tokens.

Rename Token to TokenBase and make it a superclass for TokenList and a
new Token class.  Move some of the functionality of TokenBase into Token
and TokenList.  This will make it easier to maintain separate
functionality for Token versus TokenList.

The fact that a new value was being computed each time
TokenList.group_tokens() was called caused supra-linear runtime when
token grouping was enabled.

Address by making TokenList.value a dynamically-computed property rather
than a static attribute.