Clean up `Field` trait #3643

Job	Run time
miri	3m 59s
Basic Checks	8m 38s
Measure coverage	9m 4s
Additional Builds and Concurrency Tests	8m 36s
Release builds and tests	4m 23s
sanitize (address)	5m 38s
sanitize (address, multi-threading)	4m 59s
sanitize (leak)	3m 21s
sanitize (leak, multi-threading)	3m 23s
	52m 1s

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