Compiling `rust-2018-migration` for `aarch64` and `wasm`

Hi,

I am trying to port a project to `aarch64` and `wasm` using the `rust-2018-migration` branch. As of today I receive lots of:

```
2 | use crate::vektor::x86_64::*;
| ^^^^^^ Could not find `x86_64` in `vektor`
```

Ideally, `faster` would have fallbacks for not-yet supported architectures. That way I could just write my SIMD code once using the provided API, instead of having two separate implementations.

Do you have any short-term plans of making such a fallback available for the 2018 version?

Also, while I am not a Rust expert, I have 1 - 2 days to look into this myself. If you think it's feasible to outline a solution you prefer, I'd be happy to try to help you out.