Replace Vc::array by fork from libc++
Replace Vc::array by fork from libc++

The libc++ std::array implementation uses MIT / a BSD-like license,
making it compatible with the BSD licensing used throughout the rest of
Vc. Therefore:
Replaced the Vc::array implementation that was forked from libstdc++
with the libc++ version.

* Clean up all uses of underscore prefixes, which are only allowed in
the standard library.
* Remove all _LIBCPP_ macros.
* Put the array class into the Vc namespace.
* Put the tuple interface for the array class into the std namespace.
* Replace __is_nothrow_swappable builtins with a noexcept test on
std::swap
* Preliminary documentation of the additional subscript operators.

Fixes: gh-72
Signed-off-by: Matthias Kretz <372ea4eca737a13cd010ee806d4ce7b98ee71550@kde.org>

VcDevel/Vc,VcDevel/Vc,chr-engwer/Vc,chr-engwer/Vc,VcDevel/Vc,VcDevel/Vc,chr-engwer/Vc