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.