Use sealed abstract case classes to reduce bincompat surface

[armanbilge]

sealed abstract case classes don't have apply or copy methods, but still have nice toString, hashCode, and equals.

Unfortunately they still generate unapply methods, so I've overridden those with package-private implementations to prevent the public one from being generated.

I've made the apply methods for events package-private. In ordinary use they should not be necessary, and importantly it protects our bincompat should AWS decide to add more non-optional fields, which technically they can do backwards-compatibly at any time.

Then, we can expose apply methods in the testkit like I propose doing for Context in #247. This will enable them to be mocked.

I didn't finish the DynamoDB-related stuff, I think I might like to make some bigger changes there in a followup.

[armanbilge]

• Re-encode Context and events for better compatibility and mockability #425