make OP_AELEMFAST work with negative indices

make OP_AELEMFAST work with negative indices

Use aelemfast for literal index array access where the index is in the
range -128..127, rather than 0..255.

You'd expect something like $a[-1] or $a[-2] to be a lot more common than
$a[100] say. In fact a quick CPAN grep shows 66 distributions
matching /\$\w+\[\d{3,}\]/, but "at least" 1000 matching /\$\w+\[\-\d\]/.
And most of the former appear to be table initialisations.