recognize type definitions whenever possible

```c
typedef float foo

foo main() {}
```

```
t.c:1:18: ERROR - syntax error; expected `;' before `identifier'
  typedef float foo
                   ^
t.c:3:1: ERROR - unknown type `foo'
  foo main() {}
  ^~~
t.c:3:1: ERROR - redeclaration of `foo'
  foo main() {}
  ^~~
```
