Remove inheriting from jsonld-signatures abstract classes

[tplooker]

Because jsonld-signatures is un-typed (written in JS) and this library leverages types through typescript when inheriting from an un-typed abstract based class the TS compiler is essentially blind. To improve the development experience we should sever this dependency and do one of the following

1. Define our own base classes but in typescript
2. Depend on types defined in transmutes version
3. Dont define an abstract base class and just define the remaining API's directly in the signature suite classes.

See here for an example

[kdenhartog]

This has turned into a larger issue. jsonld.js needs to be properly typed first and so does jsonld-signatures before this suite can be defined. Otherwise there's likely to be typing conflicts in this library.