Document memory layout of vector types

[danderson]

Fixes #327

---

Also contains a tiny unrelated commit, fixing a dangling link to libm in the top-level crate docs that cargo doc complained about.

[Shnatsel]

Thanks! I like the tests. We might even enforce stronger layout guarantees later, since we do align 512-bit vectors to 512 bits on all platforms AFAIK. Doesn't have to happen as part of this PR though.

It would also be nice to document that the layout of masks varies between platforms and shouldn't be relied upon.

I don't recall if our vectors are repr(C) right now, we might want to make them if they aren't (and see if that alters the layout in any way).

[DJMcNab]

Welcome!

For awareness, our docs are intended to be built with all features enabled. If you look at https://docs.rs/fearless_simd/latest/fearless_simd/#feature-flags for example, you will see that this link is resolved.

In the past, we've fixed this with a conditional compilation approach:

https://docs.rs/color/latest/src/color/lib.rs.html#75

I'm not going to die on this hill though.
If you are going to apply this fix as-is, you need to remove the duplicate link in README.md (once you've regenned it)