[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/container/optional.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include "glacier/memory/move.h"
+
+namespace glcr {
+
+template <typename T>
+class Optional {
+ public:
+  Optional() : empty_(nullptr), has_value_(false) {}
+  Optional(const T& value) : value_(value), has_value_(true) {}
+  Optional(T&& value) : value_(Move(value)), has_value_(true) {}
+  Optional(const Optional&) = default;
+  Optional(Optional&&) = default;
+  ~Optional() {
+    if (has_value_) {
+      value_.~T();
+    }
+  }
+
+  bool empty() const { return !has_value_; }
+  explicit operator bool() { return has_value_; }
+
+  const T& value() const { return value_; }
+  T&& release_value() {
+    has_value_ = false;
+    return Move(value_);
+  }
+
+  T* operator->() { return &value_; }
+  T& operator*() { return value_; }
+
+ private:
+  union {
+    T value_;
+    void* empty_;
+  };
+  bool has_value_;
+};
+
+}  // namespace glcr