Return &mut str from encode_to_slice

[elichai]

I use this crate a lot, especially in no_std or performance-sensitive environments. and I use encode_to_slice quite a lot, and usually, I need then to convert the slice into an str to pass to a serializer/formatter.
I think it makes sense that encode_to_slice will return a &mut str because it just checked all the invariants and can verify that in debug mode.

Sadly this breaks the API in the rare case where the returned () was used, so bumped to 0.5.

[elichai]

The errors are completely unrelated to this PR, I can still fix them if you prefer (although I'm not sure if changing the lint name will break older rustc versions)

[hkBst]

This looks good, but a concern may be that it would not work with #[forbid(unsafe)], which some people are interested in.