Introducing clap on tidy

[Shunpoco]

Context

Currently tidy parses paths/flags from args_os manually, and the extraction is spreading multiple files. It may be a breeding ground for bugs.

ref: https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/How.20--ci.3Dtrue.20interact.20with.20CiEnv.3F/near/543171560
(N.B. We've talked about introducing a ci flag in tidy in that thread, but I don't do it in this PR as I don't want to put multiple changes into a PR. I will introduce the flag in a coming PR.)

Changes

This PR replaces current parsing logic with clap. To confirm the new parser works fine, I introduce an unit test for it.

Build time

We've concerned about how clap increases the build time. In order to confirm the increment is acceptable, I did an experiment on CI:

• Run cargo build without cache for tidy 50 times in each environment on CI
• Calculate an average and a standard deviation from the result, and plot them

Here is the graph:

• Clap tends to increase build time ~2s. We think this is not a big problem
• Build time differs in each environment
• In some cases standard deviation are high, I suppose that busyness of CI instances affect build time