Enable support to initialize a string_t with a const char[] within M_LET.

m-string.h

@@ -1273,6 +1273,7 @@ namespace m_string {
 /* Define the OPLIST of a STRING */
 #define STRING_OPLIST                                                   \
   (INIT(string_init),INIT_SET(string_init_set), SET(string_set),        \
+   INIT_WITH(string_init_set_str),                                      \
    INIT_MOVE(string_init_move), MOVE(string_move),                      \
    SWAP(string_swap), CLEAN(string_clean),                              \
    CLEAR(string_clear), HASH(string_hash), EQUAL(string_equal_p),       \

tests/test-mstring.c

@@ -33,6 +33,9 @@ static void test1(void)
     string_set(s2, s1);
     assert(string_equal_p(s1, s2));
   }
+  M_LET((s1,"OK"),string_t) {
+    assert(string_equal_str_p(s1, "OK"));
+  }
 }
 
 static void test_utf8_basic(void)