Clean up `Field` trait #3649

Job	Run time
Measure coverage	7m 58s
Additional Builds and Concurrency Tests	8m 16s
Basic Checks	7m 17s
miri	3m 46s
Release builds and tests	6m 42s
sanitize (address)	5m 34s
sanitize (address, multi-threading)	6m 21s
sanitize (leak)	4m 38s
sanitize (leak, multi-threading)	4m 34s
	55m 6s

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