Add newtype for Claims used in tokens and validation for these

[brycx]

As suggested herein:

• https://github.com/paseto-standard/paseto-spec/blob/master/docs/02-Implementation-Guide/02-Validators.md
• https://github.com/paseto-standard/paseto-spec/blob/master/docs/02-Implementation-Guide/04-Claims.md
• https://github.com/paseto-standard/paseto-spec/blob/master/docs/02-Implementation-Guide/05-API-UX.md