Deprecate the bytes!() macro.

Replace its usage with byte string literals, except in `bytes!()` tests.
Also add a new snapshot, to be able to use the new `b"foo"` syntax.

The src/etc/2014-06-rewrite-bytes-macros.py script automatically rewrites `bytes!()` invocations into byte string literals. Pass it filenames as arguments to generate a diff that you can inspect, or `--apply` followed by filenames to apply the changes in place. Diffs can be piped into `tip` or `pygmentize -l diff` for coloring.

Closes #14646.
Closes #14881.

r? @alexcrichton