Adds iterator over struct field expressions _and_ expansions #3504

Job	Run time
Setup Build Matrix (true, ion-rust, windows, macos, ubuntu, al2-x86, al2-arm, windows, macos, ubu...	1s
Build and Test (windows, default)	6m 42s
Build and Test (windows, all)	6m 10s
Build and Test (windows, experimental-ion-hash)	4m 10s
Build and Test (windows, experimental)	6m 50s
Build and Test (macos, default)	2m 1s
Build and Test (macos, all)	2m 44s
Build and Test (macos, experimental-ion-hash)	2m 6s
Build and Test (macos, experimental)	2m 20s
Build and Test (ubuntu, default)	2m 11s
Build and Test (ubuntu, all)	4m 27s
Build and Test (ubuntu, experimental-ion-hash)	2m 5s
Build and Test (ubuntu, experimental)	2m 55s
Build and Test (al2-x86, default)	1m 20s
Build and Test (al2-x86, all)	14m 57s
Build and Test (al2-x86, experimental-ion-hash)	1m 17s
Build and Test (al2-x86, experimental)	14m 52s
Build and Test (al2-arm, default)	1m 15s
Build and Test (al2-arm, all)	14m 54s
Build and Test (al2-arm, experimental-ion-hash)	1m 12s
Build and Test (al2-arm, experimental)	2m 16s
Build and Test Confirmation	1s
	1h 36m 46s

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