Use Rust type system to make `Regmap` API safer

@raw-bin suggested this during last Saturday's call - cool idea thank you !!

> Robin Randhawa 3:09 PM
> Have you guys seen this neat crate ? https://crates.io/crates/register
> We use it here: https://github.com/rust-embedded/cortex-a

Probably best started after #254 is merged?