Add a type for server names

[jplatte]

With this + ruma/ruma-identifiers#24 we'll also be able to update the server_name variants to return references to this new type. There is probably no point in releasing the new type under a non-breaking version 0.16.1 because it will be a breaking change for the higher-level crates to switch to this new type anyway.

[jplatte]

Additionally, we should convert the new constructors to not be fallible and take a ServerNameRef as their argument.

[iinuwa]

Did this get accidentally closed or linked to the wrong commit?

[jplatte]

Yeah, once again importing the history of a sub-crate led to an issue being closed because it shares the same number as a different issue on the original repo 🙄

Thanks for noticing!