Suggest: generate `drop`, `new` and `rep` for aliased handle types

In `tests/codegen/resource-alias.wit`, we generate handle types for each alias.

```C
typedef exports_my_resources_e1_own_x_t exports_my_resources_e2_own_x_t;

typedef exports_my_resources_e1_borrow_x_t exports_my_resources_e2_borrow_x_t;
```

However, we only generate helper functions for `e1`, not for `e2`.

```C
extern void exports_my_resources_e1_x_drop_own(exports_my_resources_e1_own_x_t handle);
extern void exports_my_resources_e1_x_drop_borrow(exports_my_resources_e1_own_x_t handle);
extern exports_my_resources_e1_own_x_t exports_my_resources_e1_x_new(exports_my_resources_e1_x_t *rep);
extern exports_my_resources_e1_x_t* exports_my_resources_e1_x_rep(exports_my_resources_e1_own_x_t handle);
void exports_my_resources_e1_x_destructor(exports_my_resources_e1_x_t *rep);

```

If feels pretty strange from user perspectives that even though we are working inside interface `e2`, but we have to call functions with interface name `e1` to create `own` and `borrow` handle types.

Suggestion: we generate the same set of helper functions for aliased handle types.

Thoughts? @alexcrichton @dicej