Reinstate "Use new Svt_INVLIST for inversion lists."

This reverts commit 2e0b8fbeab3502bee36f25825c3cdd0d075c4fd3, which
reverted e0ce103ae532f9576f54a5938a24d1ee98dfb928, thus reinstating the
latter commit.  It turns out that the error being chased down was not
due to this commit.

Its original message was:

This converts inversion lists to use their own scalar type.

1 files changed, 8 insertions, 0 deletions

diff --git a/sv.h b/sv.h
index 0205f5866e..a891ce9495 100644
--- a/sv.h
+++ b/sv.h
@@ -528,6 +528,14 @@ struct xpvlv {
     char	xlv_flags;	/* 1 = negative offset  2 = negative len */
 };
 
+struct xpvinvlist {
+    _XPV_HEAD;
+    IV          prev_index;
+    STRLEN	iterator;
+    STRLEN	count;
+    bool	is_offset;	/* */
+};
+
 /* This structure works in 3 ways - regular scalar, GV with GP, or fast
    Boyer-Moore.  */
 struct xpvgv {