Describe the problem you'd like to have solved

It appears the impl package contains part of the internal implementation and should not be used by the user.
However, some of its classes are public and there is no indication to the user that they should not be used.

Describe the ideal solution

• Hide the package from javadoc
• Add a package-info.java file documenting that the package is part of the internal implementation and classes inside of it should not be used.
• Don't export the package from the module declaration, see also Add module system support #483. (Ideally in the next major version since it is a breaking change)
• Remove mentions of impl classes from API documentation:
  • NullClaim: Mentioned from
    • Header.getHeaderClaim
    • Payload.getClaim