&CORE::substr()

&CORE::substr()

This commit makes &CORE::substr callable through references and via
&ampersand syntax.

It’s a bit awkward, as we need a substr op that is flagged as hav-
ing 4 arguments *and* possibly returning an lvalue.   The code in
op_lvalue_flags wasn’t really set up for that, so I needed to flag
the op with OPpMAYBE_LVSUB in coresub_op before it gets passed to
op_lvalue_flags.  It turned out that only that was necessary, as
op_lvalue_flags does an op_private == 4 check (rather than (op_private
& 7) == 4 or some such) when checking for the 4-arg case and croak-
ing.  When the op arrives in op_lvalue_flags, it’s already flagged
OPpMAYBE_LVSUB|4 which != 4.

pp_substr is also modified to check for nulls and, if necessary,
adjust its count of how many arguments were actually passed.)