Renaming macros for ADAPT_STRUCT to deduce type or take provided attribute types.
Factoring out of adapt_struct.hpp all the FILLER_* macros details.

As of BOOST_FUSION_ADAPT_STRUCT without type deduction, there were only
simple attributes filler. Now this need more complex logic to handle both
API: the backward compatible one with type specificiation, the one without
type specification and the non-variadic one which enables omitting the type
specification by providing BOOST_FUSION_ADAPT_AUTO instead of the type.

That's why I extracted the FILLER_* macros details in a separate source
file.