[Glacier] Add a binary tree implementation.

Additionally add an optional class to return found values
in the tree. And a reference container (Ref) similar to
std::reference_wrapper to allow storing references in containers.

lib/glacier/memory/reference.h

@@ -0,0 +1,18 @@
+#pragma once
+
+namespace glcr {
+
+template <typename T>
+class Ref {
+ public:
+  Ref(T& ref) : ref_(ref) {}
+  Ref(const Ref& other) = default;
+  Ref(Ref&& other) = default;
+
+  operator T&() const { return ref_; }
+
+ private:
+  T& ref_;
+};
+
+}  // namespace glcr