Implement ClaimSet subtypes

[sjoerdvisscher]

In the jose library, unregisteredClaims is deprecated. Here is a possible new implementation that follows the suggestion in the jose library to use subtypes.

If you think this is the right approach I'll continue with rewriting the tests.

[ErinvanderVeen]

Generally looks good to me! I look forward to seeing the tests!

[ErinvanderVeen]

The Style Check CI step incorrectly succeeds with the following output:

Run nix-shell --run "bin/autoformat.sh"
error:
       … while calling the 'import' builtin

         at «string»:1:2:

            1| (import <nixpkgs> {}).bashInteractive
             |  ^

       … while calling the 'findFile' builtin

         at «string»:1:9:

            1| (import <nixpkgs> {}).bashInteractive
             |         ^

       error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)

       at «none»:0: (source not available)
will use bash from your environment
No hints

I think for now, just run bin/autoformat.sh manually. I'll create an issue that CI is broken.