Show impl for Rc

Based on rust-lang/rust-clippy#13223

Various refactoring on `clippy_dev` before upgrading rename and
splitting `clippy_lints` into multiple crates.

Some improvements:
* The working directory is set to the root clippy directory. Running
from a subdirectory was kind of supported before sometimes. Now it just
works.
* File won't be written unless they're actually updated. Most of the
time they weren't written, but a few cases slipped through.
* Buffers are reused more for the negligible speed boost.

changelog: None