This library has been running in a production codebase for a while and
can be considered stable.
There is a minor breaking change between this version and the previous
0.1.0 in that the `ValidationError` enum has gained additional variants.
Implements initial validations of token claims. The included
validations are:
* validation of token issuer
* validation of token audience
* validation that a subject is set
* validation that a token is not expired