Add support for given an argument for the initialization of a variable in M_LET

if the variable is within parenthesis:
 M_LET((a,b), string_t)
 ==> It will init a & set it to b.

For the moment, we can only init with value of the same type (through init_set)

tests/test-mcore.c

@@ -173,6 +173,9 @@ static void test_let(void)
 {
   M_LET(z, M_CLASSIC_OPLIST(testobj)) {
     testobj_add(z,z,z);
+    M_LET((zz,z), M_CLASSIC_OPLIST(testobj)) {
+      testobj_add(zz,zz,z);
+    }
     break;
     assert(0);
   }