Revert "Prevent mutable keys in Pairs, RT #128965"
Revert "Prevent mutable keys in Pairs, RT #128965"

This reverts commit dd9b760fcedcad3a9329fa76c651ca5adf0a579c.

This will be fixed more generally at a later date when there are
"a good number less problems that are affecting users who are
trying to get things done in Perl 6, rather than break stuff"

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