Add type parameter to MergingRun to limit possible merge types

[mheinzel]

Description

This is just a phantom type parameter, the Merge inside still stores a general MergeType which is not statically guaranteed to be part of the restricted type. We can also go all the way, but then we get the type parameter and IsMergeType constraints in a lot of places, e.g. on Merge.steps and the NoThunks instances (which get quite a bit more cumbersome). So I decided against it for now, but I'm happy to revisit that choice.

Snapshots can always encode the full MergeType. At first I decided to do the partial conversion in fromSnapIncomingRun, potentially throwing an exception (first commit). Later I switched it to failing in decodeVersioned, which makes the serialisation code less mechanical, but seems to fit into our error handling better. I'm open to both solutions, so let me know if you have a preference.