Add a IterationBuffer.new(iterable) candidate (#4599)
Add a IterationBuffer.new(iterable) candidate (#4599)

To facilitate initializing iteration buffers.  Makes

    IterationBuffer.new(<a b c d e f g h i j>);

about 2x as fast as:

    my $b := IterationBuffer.new;
    $b.push($_) for <a b c d e f g h i j>;
    $b
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