* Improve deserialization performance around validation and its tests
The claims validation was done via deserializing into a Map, which
implies allocations/deallocations. This was done even if the map was not
used afterwards.
This commit improves performance of the validation by never
deserializing in a `Map`, and deserializing only when necessary, to
a struct that typically only borrows from the original b64-decoded
json string.
The validation function interface change required update to the tests,
which are also made easier to read by using the `serde_json::json!`
macro.
* unrelated: fix bench compilation
f9771f7835