DEV: coccinelle: Add rule to use `istnext()` where possible

This matches both `istadv(..., 1)` as well as raw `.ptr++` uses.
diff --git a/dev/coccinelle/ist.cocci b/dev/coccinelle/ist.cocci
index c324330..97ce0a2 100644
--- a/dev/coccinelle/ist.cocci
+++ b/dev/coccinelle/ist.cocci
@@ -26,6 +26,22 @@
 struct ist i;
 @@
 
+- i = istadv(i, 1);
++ i = istnext(i);
+
+@@
+struct ist i;
+expression e;
+@@
+
+- i.ptr++;
+- i.len--;
++ i = istnext(i);
+
+@@
+struct ist i;
+@@
+
 - i.ptr != NULL
 + isttest(i)