Add RoomName struct to ruma_events::room::name.

[Frinksy]

Resolves: #159

I'm not sure if I'm going in the right direction for this PR. I've changed the constructor's signature for NameEventContent which would be a breaking change.

Also, I'm not sure if creating a RoomName type should fail if the name is an empty String.
There is one test which fails, but that is due to the fact that currently it's possible to create an empty RoomName.

[jplatte]

Yes, this was definitely expected to be a breaking change. I'll update the base branch accordingly. In the future, please select the base branch to be next for breaking changes (it might also be beneficial to use that as the starting point of your branch).

Re. empty RoomName, I think that should be disallowed. The serde attribute on the field will prevent that from being a deserialization error in that context, but being able to have an empty RoomName instance seems not useful.

[Frinksy]

I've updated the target branch and rebased my branch onto next.

Also, I've added an #[allow(clippy::exhaustive_structs)] to RoomName since I expect that it will always be just a string.

[DevinR528]

Also, I've added an #[allow(clippy::exhaustive_structs)] to RoomName since I expect that it will always be just a string.

It also makes it really awkward to use RoomName { 0: raw_string, .. } not that there would be any point in doing that because of all the convenience methods and such. But yeah, I don't think there is any tuple struct with a non_exhaustive attr.

[jplatte]

Thanks!