Bump to edition 2021 #300

Job	Run time
cross_compile_wasm	1m 9s
cross_compile_aarch64 (nightly-2022-03-24)	4m 0s
build (1.61.0, test)	3m 38s
cross_compile_aarch64 (nightly-2022-03-24, rayon)	3m 51s
build (1.61.0, benchmark)	3m 29s
cross_compile_aarch64 (nightly-2022-03-24, nightly_aarch64_neon)	3m 44s
build (1.61.0, rayon, test)	3m 29s
build (1.61.0, rayon, benchmark)	4m 13s
build (stable, test)	1m 46s
build (stable, benchmark)	1m 54s
build (stable, rayon, test)	2m 15s
build (stable, rayon, benchmark)	2m 21s
build (beta, test)	1m 34s
build (beta, benchmark)	2m 19s
build (beta, rayon, test)	1m 38s
build (beta, rayon, benchmark)	1m 56s
build (nightly, test)	1m 37s
build (nightly, benchmark)	2m 2s
build (nightly, rayon, test)	1m 52s
build (nightly, rayon, benchmark)	2m 13s
	51m 0s

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