support function call at CGenerator

Current version of libbun cannot generate function call correctly.
```$ cat call.bun
function g() {};
function f() { g(); }```

```
$ java -jar libbun-0.1.jar -t c call.bun
(call.bun:2) [error] undefined node:#ZFuncNameNode:?
        function f() { g(); }
                       ^

/* end of header */

static void g__0qqw();
static void f__0qqw();


static void g__0qqw() {
   return;
}
;

static void f__0qqw() {
   perror("(call.bun:2) [error] undefined node:#ZFuncNameNode:?\n\tfunction f() { g(); }\n\t               ^")();
   return;
}
```

This pull request supports function call.