Stop updating the lint counter with every new lint

[flip1995]

r? @Manishearth

This PR does two things:

1. Clean up the clippy_dev module a bit (first 3 commits; cc [Roadmap] Turn cargo dev into Clippys ./x.py #5394 )
2. Make the counter in the README count in steps of 50 lints. Also use a lazy_static Vec for the lint list, so no counter is required there anymore.

changelog: none

[Manishearth]

@bors r+

thanks! finally we can avoid those pesky merge conflicts!

[bors]

📌 Commit cbdf4cc has been approved by Manishearth

[bors]

⌛ Testing commit cbdf4cc with merge c211cea...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing c211cea to master...