fix(c-api) Fix how wasm_functype_t is implemented.

In `wasm.h`, `wasm_functype_t` is implemented with
`WASM_DECLARE_TYPE`, so with `WASM_DECLARE_VEC(functype, *)`. This `*`
means the C struct for the vector is defined:

```c
struct wasm_functype_vec_t {
    size_t size;
    wasm_functype_t** data;
}
```

The way we implement `wasm_functype_vec_t` in Rust is with the
`wasm_declare_vec!` macro. And it is wrong. We must use
`wasm_declared_boxed_vec!`.

lib/c-api/src/wasm_c_api/types/function.rs

@@ -90,7 +90,7 @@ impl wasm_functype_t {
     }
 }
 
-wasm_declare_vec!(functype);
+wasm_declare_boxed_vec!(functype);
 
 #[no_mangle]
 pub unsafe extern "C" fn wasm_functype_new(