Merge: Improve `Text::justify` and use it for a pretty `PerfMap::to_s`

Merge: Improve `Text::justify` and use it for a pretty `PerfMap::to_s`

This PR introduces padding services ( `pad_left` and `pad_right`) to `Text` and uses it for a cleaner print in `performance_analysis`.

Seeing how the left-pad package was critical to the node.js ecosystem motivated this PR. These methods should be useful in the long term!

Pull-Request: #2000
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>