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 splits up the needless_bool tests into `fixable.rs` and
`simple.rs`. `simple.rs` contains the code that triggers the lint
diagnostic without a suggestion.
