Upgrade for Rust 2021 #1597

Job	Run time
check	3m 15s
rustfmt	10s
test	8m 1s
clippy	3m 47s
audit	18s
integration-test-five-nodes	12m 30s
cross-aarch64	1m 46s
cross-armv7	1m 25s
integration-test-debts	16m 59s
integration-test-althea-payment	17m 11s
integration-test-eth-payments	14m 28s
integration-test-db-migration	12m 22s
integration-test-multi-exit	12m 49s
integration-test-validate-contract	12m 25s
	1h 57m 26s

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