Optimize Eulerian path algo to O(N+M)

Following this [guide](https://cp-algorithms.com/graph/euler_path.html#:~:text=This%20way%20we%20can%20achieve%20an) we can improve the current implementation of the Eulerian path https://github.com/cheran-senthil/PyRival/blob/master/pyrival/graphs/euler_walk.py by making it O(N+M) instead of O(N**2 + M)