Re-implement PR-AUC. (#7297)

* Support binary/multi-class classification, ranking.
* Add documents.
* Handle missing data.

src/common/common.h

@@ -19,6 +19,7 @@
 #include <string>
 #include <sstream>
 #include <numeric>
+#include <utility>
 
 #if defined(__CUDACC__)
 #include <thrust/system/cuda/error.h>
@@ -86,6 +87,19 @@ XGBOOST_DEVICE T1 DivRoundUp(const T1 a, const T2 b) {
   return static_cast<T1>(std::ceil(static_cast<double>(a) / b));
 }
 
+namespace detail {
+template <class T, std::size_t N, std::size_t... Idx>
+constexpr auto UnpackArr(std::array<T, N> &&arr, std::index_sequence<Idx...>) {
+  return std::make_tuple(std::forward<std::array<T, N>>(arr)[Idx]...);
+}
+}  // namespace detail
+
+template <class T, std::size_t N>
+constexpr auto UnpackArr(std::array<T, N> &&arr) {
+  return detail::UnpackArr(std::forward<std::array<T, N>>(arr),
+                           std::make_index_sequence<N>{});
+}
+
 /*
  * Range iterator
  */