[boost] [Preprocessor] Adding variadic macros support

Variadic macros would allow to support tuples better in Boost.PP, as it would not be necessary to specify the size of the tuple, and it could be possible to provide other nice features around tuples. Other possible applications could be to accept the sequence (a)(b, c, d)(e) as being the same as (a)((b, c, d))(e). If you think other things could be improved in PP with variadic macros, feel free to point out where. The question is: how should it be done? Should the new macros be named, say, BOOST_PPV_*? Should the macros in BOOST_PP_* be "overloaded" on arity (a possibility with variadic macros) so as to exhibit both the old and new interface?