Replace single RenderedEvent with RenderedSpan in Projection Codec lib

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Due to the nature of Kafka's [lack] of ordering guarantees, this PR adjusts the primary event encoding used when we feed to Kafka.

While there is an arguable benefit to retaining the earlier RenderedEvent type in situ, for clarity I'm removing it until such time as someone gets as far as deciding they absolutely need such an encoding in production, given the degree to which this will reduce throughput if ordering guarantees are to be preserved.

In other words, while I wouldn't completely rule out its return, on balance it feels more valuable to have a single unambiguous rendering, even if that might impose some complexity in some use cases.

This PR also removes an ill-thought-thru dependency on Equinox.Projection which is easy to avoid and means consumers don't depend on the projection library