rules/warningsᵒ: New, experimental ruleset

This ruleset will emit warnings, instead of doing transformations.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
rules/warningsᵒ: Prefix the warning with ';'

Since the warning goes to stdout too, prefix it with a ;, so when fed
back to Hy, it will get discarded, and thus, our result will still be a
valid program.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>