[perl #132352] Set operators unfriendly to mutable types

[via RT]

# New Ticket Created by  Zoffix Znet 
# Please include the string:  [perl #132352]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=132352 >


Set operators are a lot less useful with mutable types, like SetHash, because
even when one of the operands is a SetHash, they still return a Set, making
constructs like `∖=` or `∪=` entirely unusable:

    my $days = SetHash.new: Date.today … Date.new: '2014-04-02';
    $days ∖= $days.grep: *.key.day-of-week > 5;
    dd $days.^name; # "Set"
    $days{Date.today}:delete; # Cannot call 'DELETE-KEY' on an immutable 'Set'