Add new function string_set_n to extract a sub-string from a string.

m-string.h

@@ -242,6 +242,23 @@ string_set (string_t v1, const string_t v2)
   STRING_CONTRACT (v1);
 }
 
+static inline void
+string_set_n(string_t v, const string_t ref, size_t offset, size_t length)
+{
+  STRING_CONTRACT (v);
+  STRING_CONTRACT (ref);
+  assert (v != ref); // Not supported yet
+  if (M_UNLIKELY (ref->size == 0)) {
+    v->size = 0;
+    if (v->ptr) {
+      v->ptr[0] = 0;
+    }
+  } else {
+    assert (ref->ptr != NULL);
+    string_set_strn(v, ref->ptr + offset, length);
+  }
+}
+
 static inline void
 string_init_set(string_t v1, const string_t v2)
 {