Fix string-literal splitting to account for utf-8.
Avoid overly-cautious initializations.

This teaches NQP to distinguish 'my @a := [...];' from a 'my @a'. In
the first case, we need not initialize @a with an empty array that we
immediately throw away. Same for %h. For scalars it's less of a waste
as we allocate nothing, but still can avoid the work.