[Warnings] Removed Deprecated STD Iterator From VTR Vector

This also remove a strange alias used called "my_iter" which, according
to the comment, was used to avoid potential ambiguity in the Clang
compiler. I am not sure what this means, but I think it should be less
ambiguous when we no longer inheret from std::iterator.

libs/libvtrutil/src/vtr_vector.h

@@ -160,17 +160,16 @@ class vector : private std::vector<V, Allocator> {
      * This allows end-users to call the parent class's keys() member
      * to iterate through the keys with a range-based for loop
      */
-    class key_iterator : public std::iterator<std::bidirectional_iterator_tag, key_type> {
+    class key_iterator {
       public:
-        ///@brief We use the intermediate type my_iter to avoid a potential ambiguity for which clang generates errors and warnings
-        using my_iter = typename std::iterator<std::bidirectional_iterator_tag, K>;
-        using typename my_iter::iterator;
-        using typename my_iter::pointer;
-        using typename my_iter::reference;
-        using typename my_iter::value_type;
+        using iterator_category = std::bidirectional_iterator_tag;
+        using difference_type = std::ptrdiff_t;
+        using value_type = key_type;
+        using pointer = key_type*;
+        using reference = key_type&;
 
         ///@brief constructor
-        key_iterator(key_iterator::value_type init)
+        key_iterator(value_type init)
             : value_(init) {}
 
         /*
@@ -180,12 +179,12 @@ class vector : private std::vector<V, Allocator> {
          * we can just increment the underlying Id to build the next key.
          */
         ///@brief ++ operator
-        key_iterator operator++() {
+        key_iterator& operator++() {
             value_ = value_type(size_t(value_) + 1);
             return *this;
         }
         ///@brief decrement operator
-        key_iterator operator--() {
+        key_iterator& operator--() {
             value_ = value_type(size_t(value_) - 1);
             return *this;
         }
@@ -195,9 +194,9 @@ class vector : private std::vector<V, Allocator> {
         pointer operator->() { return &value_; }
 
         ///@brief == operator
-        friend bool operator==(const key_iterator lhs, const key_iterator rhs) { return lhs.value_ == rhs.value_; }
+        friend bool operator==(const key_iterator& lhs, const key_iterator& rhs) { return lhs.value_ == rhs.value_; }
         ///@brief != operator
-        friend bool operator!=(const key_iterator lhs, const key_iterator rhs) { return !(lhs == rhs); }
+        friend bool operator!=(const key_iterator& lhs, const key_iterator& rhs) { return !(lhs == rhs); }
 
       private:
         value_type value_;