Convert metadata_v3 to hashed env

[Bisaloo]

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[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if d69f0fb is merged into devel:

• ✔️read_blosc_lz4: 472ms -> 471ms [-0.69%, +0.16%]
• ✔️read_boolean: 11.2ms -> 11.2ms [-2.37%, +1.52%]
• ✔️read_double: 26.7ms -> 26.4ms [-2.6%, +0.35%]
• ✔️read_float16: 14.8ms -> 14.7ms [-2.97%, +2.1%]
• ✔️read_float32: 14.8ms -> 15ms [-0.83%, +3.11%]
• ✔️read_int16: 14.8ms -> 14.9ms [-0.35%, +2.11%]
• ✔️read_int32: 26.4ms -> 26.4ms [-1.26%, +1.52%]
• ✔️read_int64: 15.3ms -> 15.3ms [-1.15%, +1.15%]
• ✔️read_int8: 14.9ms -> 15ms [-1.08%, +2.33%]
• ✔️read_uint32: 16ms -> 16.1ms [-0.86%, +1.98%]
• ✔️read_uint64: 15.4ms -> 15.4ms [-1.33%, +1.89%]
• ✔️read_unicode: 8.96ms -> 8.98ms [-1.63%, +2.11%]
• ✔️read_zstd: 495ms -> 494ms [-0.78%, +0.08%]
• ✔️write_blosc_lz4: 722ms -> 742ms [-1.34%, +6.76%]
• ✔️write_bool: 13.8ms -> 13.8ms [-3.31%, +2.21%]
• ✔️write_double: 28.6ms -> 28ms [-4.22%, +0.08%]
• ✔️write_float32: 28.1ms -> 28.2ms [-1.49%, +2.43%]
• ✔️write_int16: 28.2ms -> 28.2ms [-1.84%, +1.76%]
• ✔️write_int32: 28.3ms -> 28.7ms [-1.65%, +3.98%]
• ✔️write_int8: 28.7ms -> 28.3ms [-3.39%, +0.6%]
• ✔️write_string: 12.7ms -> 12.5ms [-4.78%, +1.87%]
• ✔️write_zstd: 870ms -> 867ms [-1.32%, +0.66%]
  Further explanation regarding interpretation and methodology can be found in the documentation.