Lint "string literals".as_bytes()

[fhartwig]

I think it would make sense to suggest using byte string literals instead of calling as_bytes() on &str literals, since byte string literals are less noisy. There is a surprising amount of rust code that uses as_bytes on string literals. Some of it exists because beginners might not know that they can do b"foo" instead of "foo".as_bytes(), some of it seems to pre-date the introduction of byte-string literals.

Complication: the two aren't equivalent if the string literal contains non-ascii characters, so we cannot suggest using b"☃" instead of "☃".as_bytes(), because the former would not compile.

[llogiq]

We already have the non_ascii_unicode lint, so it should be easy enough to reuse the check here.