This feature controls whether bindgen will use the which crate to detect
the rustfmt binary. This makes which an optional dependency.

which-rustfmt is a default feature which makes this change backward compatible.

This commit changes the API of rustfmt_path to return Result<Option<Cow<PathBuf>>>.

Ok(None) is returned in the case where which is disabled and no rustfmt command is
supplied explicitly either via configuration or env variable.

Downstream code checks for the presence of None to directly return the source without
emitting an error.

…sabled