Serialize the unknown predicate as JSON on `DecidePredicateUnknown` (… #721

Job	Run time
Release	7m 43s
Cache Cabal (ubuntu-22.04, ubuntu-22.04, x86_64-linux)	13m 38s
Cache Stack	10m 49s
Cache Cabal (macos-12, macos-12, x86_64-darwin)	33m 32s
Cache Stack Haddock	5m 18s
Cache Cabal (MacM1, self-macos-12, aarch64-darwin)	7m 27s
Publish Release	3s
	1h 18m 30s

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