Use new token_list_t rather than string_list_t for macro values.

Use new token_list_t rather than string_list_t for macro values.

There's not yet any change in functionality here, (at least according
to the test suite). But we now have the option of specifying a type
for each string in the token list. This will allow us to finalize an
unexpanded macro name so that it won't be subjected to excess
expansion later.