Add PSTL internal namespace qualifications

Add PSTL internal namespace qualifications

	* include/pstl/algorithm_impl.h: Add namespace qualification.
	* include/pstl/execution_defs.h: Add namespace qualification.
	* include/pstl/execution_impl.h: Add namespace qualification.
	* include/pstl/numeric_impl.h: Add namespace qualification.
	* include/pstl/parallel_backend_tbb.h: Add namespace qualification.
	* include/pstl/unseq_backend_simd.h: Add namespace qualification.
	* include/pstl/parallel_backend_utils.h: Include <cassert>.

From-SVN: r270231