untaint hash key strings
Since we are working on deprecating and removing $SAFE for [Feature #5455],
I propose untainting all string keys used for hashes in Ruby 2.6.
It will make implementing Feature #13725 easier.
Furthermore, Perl (which I assume is the influence for tainting in Ruby) does
not taint hash keys. In fact, perlsec(1) manpage states:
"Hash keys are never tainted"