patch 8.2.0056: execution stack is incomplete and inefficient

Problem:    Execution stack is incomplete and inefficient.
Solution:   Introduce a proper execution stack and use it instead of
            sourcing_name/sourcing_lnum.  Create a string only when used.

src/kword_test.c

@@ -76,6 +76,7 @@ test_isword_funcs_utf8(void)
     int
 main(void)
 {
+    estack_init();
     test_isword_funcs_utf8();
     return 0;
 }