Implement IdentFragment for Cow<T>

[randomPoison]

This allows users to use Cow<T> with format_ident! as long as T: IdentFragment. I'm currently working on a library that frequently uses Cow<str>, which currently requires me to manually deref the Cow when using format_ident!, e.g.:

format_ident!("prefix_{}", &*my_cow);

With this change, Cow behaves "as expected", i.e. users can now do:

format_ident!("prefix_{}", my_cow);

[dtolnay]

Thanks!

[dtolnay]

Published in 1.0.6.