Make pp_splice handle nonexistent array elements

Commit ce0d59f changed AVs to use NULLs for nonexistent elements.

pp_splice needs to take that into account and avoid pushing NULLs on
to the stack.
pp_hot.c:pp_aelem: Use _NN in one spot

This av can never be null here.  av_len will already have failed an
assertion if it is.
Use defelems for (goto) &xsub calls

Before ce0d59f:

$ perl -e '++$#_; &utf8::encode'
Modification of a read-only value attempted at -e line 1.

As of ce0d59f:

$ ./perl -Ilib -e '++$#_; &utf8::encode'
Assertion failed: (sv), function Perl_sv_utf8_encode, file sv.c, line 3581.
Abort trap: 6

Calling sub { utf8::encode($_[0]) } should be more or less equivalent
to calling utf8::encode, but it is not in this case:

$ ./perl -Ilib -we '++$#_; &{sub { utf8::encode($_[0]) }}'
Use of uninitialized value in subroutine entry at -e line 1.

In the first two examples above, an implementation detail is leaking
through.  What you are seeing is not the array element, but a place-
holder that indicates an element that has not been assigned to yet.

We should use defelem magic so that what the XSUB assigns to will cre-
ate an array element (as happens with utf8::encode($_[0])).

All of the above applies to goto &xsub as well.
Put AV defelem creation code in one place
Update README copyright to 2013

For RT #119625.