Extract Serialization into a Replaceable Helper

[George Steel]

Because HTML serialisation is subjective, the asHtml() and asText methods in the fragments are just weird.

The lib should ship with a default serialisation implementation that can be easily replaced, so that instead of calling RichText::asHtml() and remembering where to put a link resolver, or having it injected "magically" or whatever, we should have an "HTML Serialiser" thus: $serialiser->serialise($richTextFragment);

I think that the serialiser implementation should be invokable and that __invoke(FragmentInterface $fragment) : string should be defined.

[George Steel]

Use netglue/prismic-client instead