Add a default constructor to TaggedUnion; resolves #608 #542

Job	Run time
windows-msvc-shared (CAPNPROTO)	7m 57s
windows-msvc-shared (FLEXBUFFERS)	6m 29s
windows-msvc-shared (AVRO)	7m 5s
windows-msvc-shared (JSON)	11m 29s
windows-msvc-shared (XML)	3m 59s
windows-msvc-shared (TOML)	4m 25s
windows-msvc-shared (PARQUET)	40m 13s
windows-msvc-shared (UBJSON)	7m 2s
windows-msvc-shared (CBOR)	8m 37s
windows-msvc-shared (MSGPACK)	4m 22s
windows-msvc (CAPNPROTO)	7m 35s
windows-msvc (TOML)	4m 38s
windows-msvc (FLEXBUFFERS)	5m 37s
windows-msvc (CBOR)	8m 13s
windows-msvc (PARQUET)	40m 8s
windows-msvc (XML)	4m 1s
windows-msvc (JSON)	11m 2s
windows-msvc-shared (YAML)	4m 15s
windows-msvc (MSGPACK)	3m 50s
windows-msvc (AVRO)	8m 12s
windows-msvc (YAML)	4m 44s
windows-msvc (UBJSON)	7m 46s
windows-msvc (benchmarks)	48m 34s
	4h 20m 13s

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