Markdown models don't have equals/hashcode implementation

[rock3r]

Since #331, equality and hashcode checks on ALL Markdown entities are meaningless, since all those models are value classes wrapping CommonMark models. CommonMark models do NOT implement equals and hashcode. This is potentially problematic for Compose usages, and many other cases.

We need to revert #331, since value classes do not support custom equals and hashcode implementations and we cannot rely on CommonMark models. We need true, content-based equals and hashcodes.

[rock3r]

@obask updates on this? I want to ship it in 0.20

[rock3r]

Fixed in #458