[Rust] See through type aliases for borrowing optimizations

As noted in https://github.com/bytecodealliance/wit-bindgen/issues/769 and https://github.com/bytecodealliance/wit-bindgen/pull/675, type aliases cause certain optimizations around borrowing to not longer be applied.

For example take the following semantically equivalent functions in wit:

```wit
borrow: func(param: list);

type param = list;
no-borrow: func(param: param);
```

Those functions will be generated as the following:

```rust
fn borrow(param: &[u8]);

type Param = Vec;
fn no_borrow(param: Param);
```

Unfortunately, we want to preserve the structure of the wit files in the generated code meaning that generating the `no_borrow` function with a `&[u8]` param instead of the `Param` alias would violate that tenant. It is unclear which should take precedent, the borrowing optimization or the desire to maintain wit structure.