rules/warningsᵒ: New, experimental ruleset

This ruleset will emit warnings, instead of doing transformations.

Requested-by: Morten Linderud <mcfoxax@gmail.com>
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>
hydiomatic.core: Bail out earlier if no transformations made

Both simplify and simplifications should bail out if the first
transformation fails, instead of trying at least twice.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
rules/control-structᵒ: Drop unnecessary (do)s from fn/defn/defun

Requested-by: Abhishek L <abhishek.lekshmanan@gmail.com>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>