Implements the `flatten` macro #3446

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)	4m 30s
Build and Test (windows, all)	7m 27s
Build and Test (windows, experimental-ion-hash)	6m 25s
Build and Test (windows, experimental)	7m 0s
Build and Test (macos, default)	1m 45s
Build and Test (macos, all)	2m 40s
Build and Test (macos, experimental-ion-hash)	2m 35s
Build and Test (macos, experimental)	2m 22s
Build and Test (ubuntu, default)	2m 12s
Build and Test (ubuntu, all)	3m 44s
Build and Test (ubuntu, experimental-ion-hash)	2m 30s
Build and Test (ubuntu, experimental)	2m 54s
Build and Test (al2-x86, default)	1m 39s
Build and Test (al2-x86, all)	2m 16s
Build and Test (al2-x86, experimental-ion-hash)	2m 3s
Build and Test (al2-x86, experimental)	2m 9s
Build and Test (al2-arm, default)	1m 35s
Build and Test (al2-arm, all)	2m 15s
Build and Test (al2-arm, experimental-ion-hash)	1m 48s
Build and Test (al2-arm, experimental)	2m 2s
Build and Test Confirmation	1s
	1h 1m 53s

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