Use concepts instead of enable_if #390

Job	Run time
windows-msvc-shared (JSON)	11m 32s
windows-msvc-shared (AVRO)	6m 30s
windows-msvc-shared (CBOR)	8m 28s
windows-msvc-shared (TOML)	3m 41s
windows-msvc-shared (CAPNPROTO)	7m 16s
windows-msvc-shared (UBJSON)	7m 7s
windows-msvc-shared (PARQUET)	39m 14s
windows-msvc-shared (MSGPACK)	4m 12s
windows-msvc-shared (FLEXBUFFERS)	5m 51s
windows-msvc-shared (YAML)	4m 27s
windows-msvc-shared (XML)	3m 59s
windows-msvc (FLEXBUFFERS)	4m 22s
windows-msvc (TOML)	3m 53s
windows-msvc (MSGPACK)	4m 34s
windows-msvc (benchmarks)	13m 52s
windows-msvc (UBJSON)	7m 1s
windows-msvc (JSON)	10m 31s
windows-msvc (YAML)	4m 9s
windows-msvc (CBOR)	8m 8s
windows-msvc (CAPNPROTO)	7m 45s
windows-msvc (AVRO)	6m 46s
windows-msvc (PARQUET)	13m 52s
windows-msvc (XML)	4m 2s
	3h 11m 12s

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