Rewrite against json_schemer

[pboling]

See https://github.com/voxpupuli/json-schema/issues/423#issuecomment-1728487278

I'm mostly opening this for visibility, as I think it makes sense to use a piggy-backed namespace like json_schemer-fuzz instead of the one used here. Since this is a rewrite and changing the underlying core dependency I'm not sure if it makes sense to keep the new code under this old namespace.

I also have no expectations that this will be merged, though I have already found and fixed several bugs.