Implement Range Iterators

This PR implements range iterators in the base containers (Vector, Map, List, Pair Set).
Given several of these data structures will be replaced by more efficient versions, having a common iterator API will make this simpler.
Iterating can be done as follows (examples):

```C++
//Vector<String>
for(const String& I: vector) {

}
//List<String>
for(const String& I: list) {

}
//Map<String,int>
for(const KeyValue<String,int>&I : map) {
	print_line("key: "+I.key+" value: "+itos(I.value));

}

//if intending to write the elements, reference can be used

//Map<String,int>
for(KeyValue<String,int>& I: map) {
	I.value = 25;
	//this will fail because key is always const
	//I.key = "hello"
}

```

The containers are (for now) not STL compatible, since this would mean changing how they work internally (STL uses a special head/tail allocation for end(), while Godot Map/Set/List do not).
The idea is to change the Godot versions to be more compatible with STL, but this will happen after conversion to new iterators have taken place.

core/templates/vector.h

@@ -187,6 +187,70 @@ public:
 		return false;
 	}
 
+	struct Iterator {
+		_FORCE_INLINE_ T &operator*() const {
+			return *elem_ptr;
+		}
+		_FORCE_INLINE_ T *operator->() const { return elem_ptr; }
+		_FORCE_INLINE_ Iterator &operator++() {
+			elem_ptr++;
+			return *this;
+		}
+		_FORCE_INLINE_ Iterator &operator--() {
+			elem_ptr--;
+			return *this;
+		}
+
+		_FORCE_INLINE_ bool operator==(const Iterator &b) const { return elem_ptr == b.elem_ptr; }
+		_FORCE_INLINE_ bool operator!=(const Iterator &b) const { return elem_ptr != b.elem_ptr; }
+
+		Iterator(T *p_ptr) { elem_ptr = p_ptr; }
+		Iterator() {}
+		Iterator(const Iterator &p_it) { elem_ptr = p_it.elem_ptr; }
+
+	private:
+		T *elem_ptr = nullptr;
+	};
+
+	struct ConstIterator {
+		_FORCE_INLINE_ const T &operator*() const {
+			return *elem_ptr;
+		}
+		_FORCE_INLINE_ const T *operator->() const { return elem_ptr; }
+		_FORCE_INLINE_ ConstIterator &operator++() {
+			elem_ptr++;
+			return *this;
+		}
+		_FORCE_INLINE_ ConstIterator &operator--() {
+			elem_ptr--;
+			return *this;
+		}
+
+		_FORCE_INLINE_ bool operator==(const ConstIterator &b) const { return elem_ptr == b.elem_ptr; }
+		_FORCE_INLINE_ bool operator!=(const ConstIterator &b) const { return elem_ptr != b.elem_ptr; }
+
+		ConstIterator(T *p_ptr) { elem_ptr = p_ptr; }
+		ConstIterator() {}
+		ConstIterator(const ConstIterator &p_it) { elem_ptr = p_it.elem_ptr; }
+
+	private:
+		const T *elem_ptr = nullptr;
+	};
+
+	_FORCE_INLINE_ Iterator begin() {
+		return Iterator(ptrw());
+	}
+	_FORCE_INLINE_ Iterator end() {
+		return Iterator(ptrw() + size());
+	}
+
+	_FORCE_INLINE_ ConstIterator begin() const {
+		return ConstIterator(ptr());
+	}
+	_FORCE_INLINE_ ConstIterator end() const {
+		return ConstIterator(ptr() + size());
+	}
+
 	_FORCE_INLINE_ Vector() {}
 	_FORCE_INLINE_ Vector(const Vector &p_from) { _cowdata._ref(p_from._cowdata); }