The `reqwest` crate depends on `base64` 0.21. However, users not
depending on the (default) `reqwest` feature may wish to use the newer
`base64` release.

This ensures that `Client` implements both traits.

This change slightly simplifies types throughout this crate by removing
a generic type parameter in many places that already have a generic type
representing a `TokenResponse`.

BREAKING CHANGES:
 - Removes `TT` type parameter from `TokenResponse` and
   `TokenIntrospectionResponse` traits trait and adds a `TokenType`
   associated type to each trait.
 - Removes (now-redundant) `TT` type parameter from `Client` and each
   `*Response` type