Overhaul event serialization in Marten 8

[jeremydmiller]

I've never liked the programming model we expose for upcasters / downcasters today, and I can't imagine it being terribly usable. For 8.0 (and Ermine), let's reconsider the whole model. Some possible ideas:

• Embed a version or content-type field(s) in the table describing the event
• Allow for an "upcasting" model that starts from the IEvent of the old model, then moves it forward to the new model. More like Wolverine's "forwarding" model
• Allow for alternative serializers like MemoryPack or MessagePack for just the event storage
• Embed "crypto shredding" capability into the way Marten reads events