chore: update to rust 2021 edition (#70) #58

Job	Run time
set-msrv	7s
clippy_check_blst	4m 25s
clippy_check_pairing	4m 35s
Checking fmt and docs	26s
macos (1.67.1)	5m 35s
macos (stable)	1m 50s
windows (x86_64-pc-windows-msvc, stable)	3m 30s
linux (i686-unknown-linux-gnu, 1.67.1, sudo apt update && sudo apt install gcc-multilib)	6m 25s
linux (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	3m 42s
linux (x86_64-unknown-linux-gnu, 1.67.1)	6m 31s
linux (x86_64-unknown-linux-gnu, stable)	1m 52s
	38m 58s

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