message: Explicitly initialize prevector::_union to avoid new warning
  Warning from gcc 7.1 is ./prevector.h:450:25: warning:
  '*((void*)(&<anonymous>)+8).prevector<28, unsigned char>::_union.prevector<28, unsigned char>::direct_or_indirect::<anonymous>.prevector<28, unsigned char>::direct_or_indirect::<unnamed struct>::indirect'
  may be used uninitialized in this function [-Wmaybe-uninitialized]
  This is a partial port of Core
diff: @@ -356,7 +356,7 @@ public:
  fill(item_ptr(0), first, last);
  }
  - prevector() : _size(0) {}
  + prevector() : _size(0), _union{{}} {}
  explicit prevector(size_type n) : _size(0) { resize(n); }
  explicit prevector(size_type n, const T &val = T()) : _size(0)
  {
  