use const generics

[lelongg]

use const generics with arrayvec instead of generic-array for no_std

tests are passing

[regexident]

Hi @lelongg and thanks for taking the time to contribute to this project!

I migrated the median filter to const generics as part of the signalo project a while back, but apparently forgot to make the same change to this separate project, too. 🤦🏻‍♂️

I hope you understand that for the sake of consistency I'd prefer to keep both implementations in sync. I wouldn't want to just close this PR though, so please feel free to update this PR with the code from signalo and I'll be happy to merge it!

Sorry for the inconvenience! 😕