Clean up `Field` trait #3644

Job	Run time
Basic Checks	7m 18s
Additional Builds and Concurrency Tests	7m 40s
miri	3m 46s
Release builds and tests	5m 27s
sanitize (address)	6m 0s
sanitize (address, multi-threading)	4m 38s
Measure coverage	8m 5s
sanitize (leak)	11m 19s
sanitize (leak, multi-threading)	3m 48s
	58m 1s

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