Add the "implicit_clone" lint.

Check implicit_clone section of Clippy documentation for details;

https://rust-lang.github.io/rust-clippy/master/index.html#implicit_clone

This adds, and addresses, the following Clippy configuration to crates;

#![deny(clippy::implicit_clone)]

And moves related line within maint/add_warning.py file. My intent is to mitigate extra edits after merging, so please let me know if I need to do this last bit differently.