(De)?Serialization should report problem in a clean manner if DU does not have unique Event Type names

[bartelink]

In EncodedEvent.serializeUnionByCaseItemType and its inverse there is a reliance on a Seq.exactlyOne.

This should be replaced with a cached DU-level consistency verification with a clear message using a technique such as that in

FunDomain/FunDomain.Persistence/Serialization.fs

Lines 194 to 200 in 44ad21d

	let converters<'a> =
	let valueConverters =
	getValues typeof<'a>.Assembly
	|> Seq.map valueConverter
	|> Seq.toList
	[ unionConverter; optionConverter]
	@ valueConverters

allowing the actual lookup to just be on a find-first basis.