The store only works for values that are String.

https://github.com/jdockerty/sqrl/blob/9055210006cc31bd6996d2dc5ce22ea5afd3522b/src/store.rs#L96-L98

This should be converted to storing generic values, keeping keys as String I think is okay for now.

As a reference, I believe we should be able to look in the standard library at how this is implemented for HashMap or any other K-V library (e.g. DashMap).