Make my $a; $a.AT-POS(0) = 42 about 15% faster
Make my $a; $a.AT-POS(0) = 42 about 15% faster

By simplifying the code in Any.AT-POS itself, and also inside of the
$!whence.  This also makes my $a; $a[0] = 42 faster, but less so
because of postcircumfix:<[]> overhead.

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