add op_sibling_splice() fn and make core use it

The op_sibling_splice() is a new general-purpose OP manipulation
function designed to edit the children of an op, in an analogous
manner in which the perl splice() function manipulates arrays.

This commit also edits op.c and a few other places to remove most direct
manipulation of op_sibling, op_first and op_last, and replace that with
calls to op_sibling_splice().

This has two advantages. First, by using the one function consistently
throughout, it makes it clearer what a particular piece of of code is
doing, rather than having to decipher lots of of ad-hoc

    cLISTOPo->op_first = OP_SIBLING(kid);

style stuff. Second, it will make it easier to later add a facility for
child OPs to find their parent, since the changes now only need to be made
in a few places.

In theory this commit should make no functional change to the code.

1 files changed, 1 insertions, 0 deletions

diff --git a/embed.h b/embed.h
index 37c5b20439..a36245d28f 100644
--- a/embed.h
+++ b/embed.h
@@ -418,6 +418,7 @@
 #define op_refcnt_lock()	Perl_op_refcnt_lock(aTHX)
 #define op_refcnt_unlock()	Perl_op_refcnt_unlock(aTHX)
 #define op_scope(a)		Perl_op_scope(aTHX_ a)
+#define op_sibling_splice(a,b,c,d)	Perl_op_sibling_splice(aTHX_ a,b,c,d)
 #define pack_cat(a,b,c,d,e,f,g)	Perl_pack_cat(aTHX_ a,b,c,d,e,f,g)
 #define packlist(a,b,c,d,e)	Perl_packlist(aTHX_ a,b,c,d,e)
 #define pad_add_anon(a,b)	Perl_pad_add_anon(aTHX_ a,b)