The following reworks the dependency engine to bring in a real
context.Context. The fallout of this means that we decouple the
concept of cancelation and where you get the dependency from. In
doing so, context has become snapshot. The original comments
around this type just state that it is a snapshot, so we might as
well just call it that.
The dependency Context becomes split. Removing the cross cutting
concerns. The aborting of a worker in the start now becomes
context.Context and the retrival of a dependency is now a Container.
Currently we're just creating a adhoc context.Context, but in the
next commit, we'll tie this to the tomb directly, ensuring that upon
destruction of the engine, all manifolds will be killed.

The context creation is now tied to the lifecycle of the tomb. Doing
this ensures that when the engine is killed, then the context is also
killed. It worth noting that cancel is idempotent so, calling it multiple
times does nothing. As the main loop on tomb death is also attempting
to cancel the context of the snapshot worker, it essentially becomes a
no-op.

Renaming Container (DI) to Getter to prevent any confusion over what
a container is in this context.

This is a breaking change, so we should bump from v3 to v4.