Add property testing where appropriate.

[aatxe]

In an effort to improve testing quality, I think it would be a good idea to incorporate some property testing. The most apparent inspiration for this to me was thinking a bit about the new format stripping trait that @freddyb contributed: In particular, it should definitely be the case that if is_formatting is false, strip_formatting should be a no-op. There are almost certainly other places where property testing could prove useful (probably most of them related to parsing). We should use proptest.

• [ ] Survey for additional sites where property testing would be useful.
• [ ] Proptest irc::proto::colors::FormattedStringExt.
• [ ] Proptest irc::proto::prefix::Prefix.