treewide: future-proof frozen_string_literal changes
Once again Ruby seems ready to introduce more incompatibilities
and force busywork upon maintainers[1]. In order to avoid
incompatibilities in the future, I used a Perl script[2] to
prepend `frozen_string_literal: false' to every Ruby file.
Somebody interested will have to go through every Ruby source
file and enable frozen_string_literal once they've thoroughly
verified it's safe to do so.
[1] https://bugs.ruby-lang.org/issues/20205
[2] https://yhbt.net/add-fsl.git/
74d7689/s/?b=add-fsl.perl