Allow chaining backends

Author: ldionne

libcxx/include/CMakeLists.txt

@@ -72,22 +72,7 @@ set(files
   __algorithm/partition_point.h
   __algorithm/pop_heap.h
   __algorithm/prev_permutation.h
-  __algorithm/pstl_any_all_none_of.h
-  __algorithm/pstl_copy.h
-  __algorithm/pstl_count.h
-  __algorithm/pstl_equal.h
-  __algorithm/pstl_fill.h
-  __algorithm/pstl_find.h
-  __algorithm/pstl_for_each.h
-  __algorithm/pstl_generate.h
-  __algorithm/pstl_is_partitioned.h
-  __algorithm/pstl_merge.h
-  __algorithm/pstl_move.h
-  __algorithm/pstl_replace.h
-  __algorithm/pstl_rotate_copy.h
-  __algorithm/pstl_sort.h
-  __algorithm/pstl_stable_sort.h
-  __algorithm/pstl_transform.h
+  __algorithm/pstl.h
   __algorithm/push_heap.h
   __algorithm/ranges_adjacent_find.h
   __algorithm/ranges_all_of.h
@@ -569,18 +554,19 @@ set(files
   __numeric/iota.h
   __numeric/midpoint.h
   __numeric/partial_sum.h
-  __numeric/pstl_reduce.h
-  __numeric/pstl_transform_reduce.h
+  __numeric/pstl.h
   __numeric/reduce.h
   __numeric/saturation_arithmetic.h
   __numeric/transform_exclusive_scan.h
   __numeric/transform_inclusive_scan.h
   __numeric/transform_reduce.h
   __pstl/backend_fwd.h
+  __pstl/backends/default.h
   __pstl/backends/libdispatch.h
   __pstl/backends/serial.h
   __pstl/backends/std_thread.h
   __pstl/configuration.h
+  __pstl/configuration_fwd.h
   __pstl/cpu_algos/any_of.h
   __pstl/cpu_algos/cpu_traits.h
   __pstl/cpu_algos/fill.h
@@ -590,13 +576,7 @@ set(files
   __pstl/cpu_algos/stable_sort.h
   __pstl/cpu_algos/transform_reduce.h
   __pstl/cpu_algos/transform.h
-  __pstl/defaults.h
-  __pstl/defaults/find_if_family.h
-  __pstl/defaults/for_each_family.h
-  __pstl/defaults/merge_family.h
-  __pstl/defaults/stable_sort_family.h
-  __pstl/defaults/transform_family.h
-  __pstl/defaults/transform_reduce_family.h
+  __pstl/dispatch.h
   __pstl/run_backend.h
   __random/bernoulli_distribution.h
   __random/binomial_distribution.h