Mark all classes as readonly

[Slamdunk]

Fix #1086

This PR needs a new 6.0.x branch

[lcobucci]

The changes are good, thanks.

I personally don't like the factory method Builder::new() but it's a necessary evil to ensure that we don't have people injecting crazy stuff there.

[Slamdunk]

I personally don't like the factory method Builder::new()

This concern is easy to address with new Headers, Header, Claims, RegisteredClaims, UnregisteredClaim, RegisteredClaim objects and so on, but it would probabily be a bit overkill