[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/ref_ptr.h

@@ -51,6 +51,8 @@ class RefPtr {
   T* get() const { return ptr_; };
   T& operator*() const { return *ptr_; }
   T* operator->() const { return ptr_; }
+
+  bool empty() const { return ptr_ == nullptr; }
   operator bool() const { return ptr_ != nullptr; }
 
   bool operator==(decltype(nullptr)) const { return (ptr_ == nullptr); }

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