Constant fedimint_core::invite_code::BECH32_HRP

const BECH32_HRP: Hrp;

We can represent client invite code as a bech32 string for compactness and error-checking

Human readable part (HRP) includes the version

[ hrp (4 bytes) ] [ id (48 bytes) ] ([ url len (2 bytes) ] [ url bytes (url len bytes) ])+