[perl #123445] More generic arguments should be acceptable for blocks supplied as closure parameters

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# New Ticket Created by  Rob Hoelz 
# Please include the string:  [perl #123445]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=123445 >


For example:

sub doing-stringy-thing(&action:(Str)) {
  action 'Hello';
}

The following fails:

doing-stringy-thing(-> $value {});

Even though $value can be any Mu (which all Strs are), the above code doesn't 
type check.