Add a default constructor to TaggedUnion; resolves #608 #541

Job	Run time
windows-msvc-shared (JSON)	11m 25s
windows-msvc-shared (MSGPACK)	3m 58s
windows-msvc-shared (FLEXBUFFERS)	6m 2s
windows-msvc-shared (PARQUET)	40m 41s
windows-msvc-shared (CAPNPROTO)	7m 48s
windows-msvc-shared (AVRO)	8m 43s
windows-msvc-shared (CBOR)	9m 21s
windows-msvc-shared (TOML)	4m 27s
windows-msvc-shared (XML)	3m 57s
windows-msvc-shared (YAML)	4m 26s
windows-msvc-shared (UBJSON)	7m 17s
windows-msvc (JSON)	11m 16s
windows-msvc (CBOR)	8m 14s
windows-msvc (AVRO)	7m 32s
windows-msvc (CAPNPROTO)	7m 52s
windows-msvc (YAML)	4m 29s
windows-msvc (TOML)	4m 13s
windows-msvc (FLEXBUFFERS)	6m 0s
windows-msvc (PARQUET)	41m 13s
windows-msvc (XML)	4m 2s
windows-msvc (UBJSON)	10m 42s
windows-msvc (benchmarks)	50m 39s
windows-msvc (MSGPACK)	3m 52s
	4h 28m 9s

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