Make `Hrp::len()` const fn

I'm creating a wrapper around `Hrp` that constraints its length to something much more reasonable than 83 characters and use `Hrp::len()` to check the length, but can't make constructor of my wrapper `const fn` because this method isn't const.

Other methods like `Hrp::as_bytes()`, `Hrp::as_str()` and probably some others could be made `const fn` too.