multiple moonbit bindgen issues

### Environment
- wit-bindgen-cli: 0.47.0
- OS: Ubuntu 22.04.5 LTS (x86_64)

Run `wit-bindgen moonbit --out-dir OUT /tmp/poc.wit` and `moon check --source-dir OUT`.

### Sub-issue 1

#### Reproduction
```
package local:mb-async-root;

world w {
export foo: async func();
}
```

### Sub-issue 2

#### Reproduction
```
package local:demo;

world w {
import foo: async func(x: tuple);
}
```

### Sub-issue 3

#### Reproduction
```
package local:demo;

world w {
flags big {
flag0,
flag1,
flag2,
flag3,
flag4,
flag5,
flag6,
flag7,
flag8,
flag9,
flag10,
flag11,
flag12,
flag13,
flag14,
flag15,
flag16,
flag17,
flag18,
flag19,
flag20,
flag21,
flag22,
flag23,
flag24,
flag25,
flag26,
flag27,
flag28,
flag29,
flag30,
flag31,
flag32,
}

export foo: func() -> big;
}
```

### Sub-issue 4

#### Reproduction
```
package local:demo;

interface i {
flags f {
a,
}

record f-flag {
}
}

world w {
export i;
}
```

### Sub-issue 5

#### Reproduction
```
package local:demo;

interface res {
resource foo {
drop: func();
}
}

world w {
export res;
}
```