Set up Miri tests in CI #173
Description
As mentioned in #159:
It would be good if you could maybe open an issue about adding miri tests to fearless_simd so we don't forget about it, but I don't think it has to happen in this PR.
In order to achieve good performance, the library has to contain quite a lot of transmutes. While their usage should be sound and I've manually verified this, it would be good to automatically verify it as well.