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

This replaces `if (i.len > e) i.len = e;` by `isttrim(i, e)`.
diff --git a/dev/coccinelle/ist.cocci b/dev/coccinelle/ist.cocci
index 5b6aa6b..7e9a6ac 100644
--- a/dev/coccinelle/ist.cocci
+++ b/dev/coccinelle/ist.cocci
@@ -46,6 +46,14 @@
 
 @@
 struct ist i;
+expression e;
+@@
+
+- if (\(i.len\|istlen(i)\) > e) { i.len = e; }
++ i = isttrim(i, e);
+
+@@
+struct ist i;
 @@
 
 - i.ptr != NULL