Added rfl::DefaultVal, resolves #534 #362

Job	Run time
windows-msvc-shared (AVRO)	3m 7s
windows-msvc-shared (JSON)	2m 52s
windows-msvc-shared (CBOR)	1m 55s
windows-msvc-shared (MSGPACK)	3m 20s
windows-msvc-shared (YAML)	3m 34s
windows-msvc-shared (CAPNPROTO)	5m 2s
windows-msvc-shared (FLEXBUFFERS)	5m 7s
windows-msvc-shared (XML)	2m 50s
windows-msvc-shared (PARQUET)	36m 26s
windows-msvc-shared (TOML)	2m 21s
windows-msvc-shared (UBJSON)	2m 7s
windows-msvc (YAML)	1s
windows-msvc (UBJSON)	1s
windows-msvc (benchmarks)	1s
windows-msvc (CAPNPROTO)	1s
windows-msvc (MSGPACK)	1s
windows-msvc (FLEXBUFFERS)	1s
windows-msvc (CBOR)	1s
windows-msvc (PARQUET)	1s
windows-msvc (XML)	1s
windows-msvc (AVRO)	1s
windows-msvc (JSON)	1s
windows-msvc (TOML)	1s
	1h 8m 53s

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