Deprecate the bytes!() macro

[SimonSapin]

Depends on #14880. Blocks #14646.

The bytes!() macro is to be removed soon. Using it should cause a compile-time warning the recommends using the new b"foo" byte string literal syntax instead (#14880).

I’d submit a PR (or make it part of #14880), but I don’t know how to do this. Since procedural macros are not defined by a single item, there is nowhere to add #[deprecated]. I suspect there is something to add around here, but any pointer is appreciated.

cc @brson

[alexcrichton]

You'd probably take a similar route to the old fmt! macro, but use span_warn instead of span_err

[SimonSapin]

Actually, do we care about doing this rather than just remove it?

[SimonSapin]

Fixed in #15025.