Until now, there were some simple host checks when verifying the HTTP sig,
meant to forbid hosts that are IP addresses, local hosts, and maybe other weird
cases. These checks moved to Network.FedURI, so now FedURIs in general aren't
allowed to have such hosts. The host type is still `Text` though, for now.
Using a dedicated type allows to record in the type the guarantees that we
provide, such as scheme being HTTPS and authority being present. Allows to
replace ugly `fromJust` and such with direct field access.
This is a lot of code, better save now than sorry later when something
gets deleted by mistake.
Either way, the code will move later - once tested and organized
properly - into its own package.
