Consider implementing Serialize/Deserialize for type Locale #6824
Description
We currently have an impl for LanguageIdentifier but not Locale. This discourages people from using Locale: #6810
In my opinion, we should implement serde on both or neither. Since we've had it on LanguageIdentifier since #89, we should probably just add it to Locale.
Please note that this is not just slapping #[derive(Serialize)]; the impl should look similar to the LanguageIdentifier impl, which does the same string serialize/parse that we currently recommend be done in userland.