Add benchmark tests to path/posix

I have written some benchmark tests to `push`, `push_many`, `join`,
`join_many` and `ends_with_path`.
auto merge of #10691 : g3xzh/rust/benchm, r=cmr

I have written some benchmark tests to `push`, `push_many`, `join`,
`join_many` and `ends_with_path`.

Let me know what you think (@cmr).
Thanks in advance.