Rename include_bin! to include_bytes!

According to [RFC 344][], methods that return `&[u8]` should have names
ending in `bytes`. Though `include_bin!` is a macro not a method, it
seems reasonable to follow the convention anyway.

We keep the old name around for now, but trigger a deprecation warning
when it is used.

[RFC 344]: https://github.com/rust-lang/rfcs/blob/master/text/0344-conventions-galore.md

[breaking-change]