Moving `DecodeSegement` to `Parser` (#278)

* Moving `DecodeSegement` to `Parser`

This would allow us to remove some global variables and move them to parser options as well as potentially introduce interfaces for json and b64 encoding/decoding to replace the std lib, if someone wanted to do that for performance reasons.

We keep the functions exported because of explicit user demand.

* Sign/Verify does take the decoded form now