Rework Path to Implement `ReadOnlyList`

[Chris3606]

Given modern C# supporting indices, ranges, etc; it would make sense for path to implement ReadOnlyList. GetStepWithStart is a very rare exception to the rule and that function could remain as is, however I propose the following API:

class Path : IReadOnlyList<Point>
{
     public Point this[int index] => /* Index of point not including start */
     public int Count => /* Length not including start */
     // GetStepWithStart could remain, so could Length.  GetStep itself could be removed or marked as deprecated.
}