xs_vec_len_2d returns the length of a 2D vector
xs_vec_distance returns the distance between two vectors
xs_vec_distance_2d returns the distance between two 2D vectors
xs_vec_add_scaled and xs_vec_sub_scaled do two operations
in one for convenience and performance. They add/subtract a vector
scaled by a scalar to another vector. Very useful when working with
unit vectors.
