side effect for `enum_variants`:
use .first() instead of .get(0) in enum_variants lint
move to_camel_case to str_util module
move module, enum and struct name repetitions check to a single file `item_name_repetitions`
rename enum_variants threshold config option
This makes sure that the tests in clippy_utils are run in CI.
When looking into this I discovered that two tests were failing and
multiple doc tests were failing. This fixes those tests and enables a
few more doc tests.
