Potential namespace conflict with heapless crate

[franziskuskiefer]

As pointed out by @cryptojedi the U8 etc. namespace conflicts with relatively popular crates such as https://github.com/japaric/heapless.

[phayes]

There's are several other crates that also define U8 and friends in various places.

I don't think this is an issue, and one can simply

use secret_integers::U8 as secret_U8;