In my opinion, if uppercase text is not defined for a human-readable representation of a bech32 address, then the data type used to represent this should not be able to contain uppercase characters. So, instead of using a type synonym type HRP = Text, we should use a newtype with a smart contractor that performs any necessary conversions (such as lower-casing) upon construction. This avoids requiring that every function, that takes a HRP as an argument, do this manually.

Technically the HRP is defined as case-insensitive, but there is nothing to prevent it from being case-preserving, so it seems correct to me to use Text instead of a custom type.