[glacier] Move Pair to glacier.

lib/glacier/container/pair.h

@@ -0,0 +1,17 @@
+#pragma once
+
+namespace glcr {
+
+template <typename T, typename U>
+class Pair {
+ public:
+  Pair(const T& first, const U& second) : first_(first), second_(second) {}
+  T& first() { return first_; }
+  U& second() { return second_; }
+
+ private:
+  T first_;
+  U second_;
+};
+
+}  // namespace glcr