Use unindent as extension function

[la10736]

It would be great to have an extension function for unindent(). I'm using it in tests and use unindent() as postfix instead of function call improve test readability:

assert_eq!(tests, r#"
                    case_1_1_1
                    case_1_1_2
                    case_1_2_1
                    case_1_2_2
                    case_2_1_1
                    case_2_1_2
                    case_2_2_1
                    case_2_2_2"#.unindent())

It is better than surround string by unindent() function.

Just I'm not sure if is better to have two different traits for unindent and unindent_bytes or use just one for both. In the last case I need to write every single implementation (&str, String, &[u8]...) but in the first case I can do just two impl for AsRef<str> and AsRef<[u8]>.

If it is ok for you I can implement it...

[dtolnay]

Thanks, I would accept a PR for this.

[la10736]

Do you prefer just one trait or two?

[dtolnay]

I would prefer one extension trait called unindent::Unindent.