<execution>: Consider separating type and data member declarations

[CaseyCarter]

As discussed here, some class definitions in <execution> (e.g.,

STL/stl/inc/execution

Lines 835 to 837 in 1980e1a

	using _Target_diff = _Iter_diff_t<_RanIt>;
	_Unwrapped_t<_RanIt> _Start_at;
	using _Chunk_type = _Iterator_range<_Unwrapped_t<_RanIt>>;

) unconventionally intermix type and data member definitions. It would be nice if these were updated to our conventional "types - blank line - data members - blank line - member functions" layout for structs.