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