Add vectorization sample.

[DanAlbert]

This adds a new generic vectorization sample to replace hello-neon. Most importantly, it covers non-Neon options for SIMD. One of the useful things it shows, for example, is that there's actually no reason to write SIMD code the way that hello-neon does any more.

This also solves a much simpler problem (small matrix multiplication), which makes it easier to see how to deal with the SIMD features rather than figuring out what a FIR filter is.

Finally, this sample benchmarks each of the implementations so it's obvious what is and isn't worth doing. I was sort of surprised that auto-vectorization didn't do better, and was pleased to learn that there's no reason at all to write Neon intrinsics.

I'll delete hello-neon after this merges and I've fixed up the doc links.

#1011

[DanAlbert]

Currently a draft because I seem to have encountered an AGP bug that makes cross-module dependencies flaky, so I may need to deal with base in a different way...

The code itself wouldn't change to accommodate that though. Aside from the todo about checking .at() vs operator[] (which might actually be significant here), the code is, I think, done.

[DanAlbert]

Added a fair amount of content to the readme for explaining FMV (to the best of my knowledge, anyway, and since I learned it all just now it might be wrong). That probably belongs on DAC rather than here, but it can go here for now.