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

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

```c
struct wasm_memorytype_vec_t {
    size_t size;
    wasm_memorytype_t** data;
}
```

The way we implement `wasm_memorytype_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/memory.rs

@@ -48,7 +48,7 @@ impl wasm_memorytype_t {
     }
 }
 
-wasm_declare_vec!(memorytype);
+wasm_declare_boxed_vec!(memorytype);
 
 #[no_mangle]
 pub unsafe extern "C" fn wasm_memorytype_new(limits: &wasm_limits_t) -> Box<wasm_memorytype_t> {