1.2.20

• added ASet.ofListTree and ASet.ofSetTree
• added AList.mapToASet
• added AMap.ofASetMapped/ofASetMappedIgnoreDuplicates (optimized ASet.groupBy)
• added MapNode/SetNode addInPlace helpers
• added SetNode.head
• added special implementation for ASet.filterA
• added special implementation for ASet.union with one set constant
• improved AList.append
• changed internal tuples/options to value types
• changed Index garbage collection to run in finalizer
• preferred using struct enumerators
• avoided using active patterns to match set operations
• updated to net 8.0
• updated Fable to 4.22.0
• fixed race condition in Index