================ PSTL integration

The PSTL (or Parallel STL) project is quite complex in its current form and does not provide everything that libc++ requires, for example _LIBCPP_HIDE_FROM_ABI or similar annotations and including granularized headers. Furthermore, the PSTL provides various layers of indirection that make sense in a generic implementation of the parallel algorithms, but are unnecessarily complex in the context of a single standard library implementation. Because of these drawbacks, we decided to adopt a modified PSTL in libc++. Specifically, the goals of the modified PSTL are

• No <__pstl_algorithm> and similar glue headers -- instead, the implementation files are included directly in <algorithm> and friends.
• No <pstl/internal/algorithm_impl.h> and <pstl/internal/algorithm_fwd.h> headers and friends -- these contain the implementation and forward declarations for internal functions respectively. The implementation lives inside <__algorithm/pstl_any_of.h> and friends, and the forward declarations are not needed inside libc++.
• No <pstl/internal/glue_algorithm_defs.h> and <pstl/internal/glue_algorithm_impl.h> headers and friends -- these contain the public API. It lives inside <__algorithm/pstl_any_of.h> and friends instead.
• The headers implementing backends are kept with as few changes as possible to make it easier to keep the backends in sync with the backends from the original PSTL.
• The configuration headers __pstl_config_site.in and pstl_config.h are removed, and any required configuration is done inside __config_site.in and __config respectively.
• libc++-style tests for the public PSTL API