experimental: \s in regexp matches \cK

According to perlexperiment, \s matching \cK is experimental as of 5.18.0.

I don't see it framed that way in perl5180delta.  It is, though, in
perlrecharclass.  There, it has this comment:

  it could be backed out in v5.20 or v5.22 if experience indicates that it
  breaks too much existing code

So far, I feel like we're A-OK to call it gold in 5.20.  If we'd rather wait
until 5.22, though, I will make a ticket hung on "blocking 5.22" to remind us
when we get there.

Thoughts?

-- 
rjbs

• experimental: \s in regexp matches \cK by Ricardo Signes
• Re: experimental: \s in regexp matches \cK by Karl Williamson