simplify by: depth + prefix.bit_len()== point

[zhiqiangxu]

The test shows bit_len() == point - depth is always true as long as point >= depth.

So depth + prefix.bit_len() is the same as depth + point - depth, which is depth.

(if point < depth, PathSegment::from_path will panic anyway.)

This PR also fixed the calculation of max_bit_len.