Allow adapters to process types that deref to their vertex type.

[obi1kenobi]

Before this PR, adapter implementations were unnecessarily strict: they required that the DataContext contain specifically their Vertex type. They would refuse to work in a situation where the DataContext contained a type that within it contained a Vertex i.e. could dereference to a Vertex upon request. That made it harder than necessary to expose adapters across programming languages, or use them as building blocks that can combine their functionality.

This PR changes the Adapter trait to allow any DataContext<V> representation, so long as that V type implements AsVertex<Self::Vertex> and has an appropriate lifetime for the adapter. When the adapter needs to look up the active_vertex() and get an Option<Self::Vertex> back, it can do so via the AsVertex trait which handles the conversion. AsVertex has a blanket impl for all Debug + Clone (i.e. valid Adapter::Vertex) types, so implementations should be able to upgrade to the new API by just changing the Adapter fn signatures and without implementation changes.