Change JSON serialization format

[jspenger]

This PR implements a custom serialization format that does not depend on the uPickle default format.

• Update tests to work with new serialization format
• Bump uPickle version: "3.3.0" -> "4.4.3"
• Replace the Packed spore representation with AST spore representation: now contains the value to be serialized whereas previously it was the serialized value
• Remove Spore[ReadWriter[EmptyTuple]]
• Add tests for serialization format

Other notes: I had to change the Scala Native version from 0.5.10 to 0.5.9 as the GitHub Actions runners otherwise (on 0.5.10) would frequently hang.

Here are some before/after examples:

Object builder:

Before:

{"$type":"spores.Packed.PackedObject","className":"spores.SporeBuilderTestsDefs$Predicate$"}

After:

{"tag":"Body","kind":0,"className":"spores.SporeBuilderTestsDefs$Predicate$"}

Class builder:

Before:

{"$type":"spores.Packed.PackedClass","className":"spores.SporeClassBuilderTestsDefs$Predicate"}

After:

{"tag":"Body","kind":1,"className":"spores.SporeClassBuilderTestsDefs$Predicate"}

Lambda builder:

Before:

{"$type":"spores.Packed.PackedLambda","className":"spores.jvm.SporeLambdaTestsDefs$Lambda$1"}

After:

{"tag":"Body","kind":2,"className":"spores.jvm.SporeLambdaTestsDefs$Lambda$1"}

With environment builder:

Before:

{"$type":"spores.Packed.PackedWithEnv","packed":{"$type":"spores.Packed.PackedObject","className":"spores.SporeBuilderTestsDefs$HigherLevelFilter$"},"packedEnv":{"$type":"spores.Packed.PackedEnv","env":"{\"$type\":\"spores.Packed.PackedObject\",\"className\":\"spores.SporeBuilderTestsDefs$Predicate$\"}","rw":{"$type":"spores.Packed.PackedObject","className":"spores.ReadWriters$SporeRW$"}}}

After (it is no longer necessary to escape by ""):

{"tag":"WithEnv","fun":{"tag":"Body","kind":0,"className":"spores.SporeBuilderTestsDefs$HigherLevelFilter$"},"env":{"tag":"Val","ev":{"tag":"Body","kind":0,"className":"spores.ReadWriters$SporeRW$"},"value":{"tag":"Body","kind":0,"className":"spores.SporeBuilderTestsDefs$Predicate$"}}}