This will allow moving some filters to run before the tokens are
grouped.

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.

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

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