[perl #129334] Backtrace.map is dead code

[via RT]

# New Ticket Created by  Zoffix Znet 
# Please include the string:  [perl #129334]
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# <URL: https://rt.perl.org/Ticket/Display.html?id=129334 >


This .map multi in Backtrace.pm never gets acccessed: 
https://github.com/rakudo/rakudo/blob/553cedb679f0f191d2f7341e05033505cb20ac7a/src/core/Backtrace.pm#L282

    <Zoffix> m: Backtrace.new.map: {...}
    <camelia> rakudo-moar 553ced: OUTPUT«Ambiguous call to 'map'; these 
signatures all match:␤:(\SELF: &block;; :$label, :$item, *%_)␤:(Backtrace:D $: 
$block, *%_)␤  in block <unit> at <tmp> line 1␤␤»

It seems like merely changing $block to &block in the signature would fix the 
issue, but I'm unsure whether :$label and :$item need to be added to it as well.

Mentioned on IRC as well: 
http://irclog.perlgeek.de/perl6-dev/2016-09-22#i_13260484