experimental: "our $attribute :unique"

"our" variables, in 5.8, could start taking a ":unique" attribute for use with
data sharing between threads.

This was made into a no-op in 5.10 and deprecated in 5.12 (perlexperiment's
claims to the contrary).  I made a branch, rjbs/not-unique, in which to finally
remove any reference to the attribute from core.  It will require synchronizing
with the threads dist.

1) Do we want to do this, or are we holding hope for the restorating of some
   way to restore this attribute?

2) threads is marked upstream blead, but it seems to generally be released on
   CPAN first, then merged into blead; do we want to change upstream?

3) Any other notes for the experiment update?

-- 
rjbs

• experimental: "our $attribute :unique" by Ricardo Signes
• Re: experimental: "our $attribute :unique" by Elizabeth Mattijsen