use new arith api (#5) #15

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	22s
build (1.81.0, wasm32-unknown-unknown)	22s
cross (armv7-unknown-linux-gnueabihf, 1.81.0)	1m 41s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 36s
build (stable, thumbv7em-none-eabi)	30s
doc	57s
cross (aarch64-unknown-linux-gnu, 1.81.0)	1m 22s
build (stable, wasm32-unknown-unknown)	19s
test (i686-unknown-linux-gnu, 1.81.0, sudo apt update && sudo apt install gcc-multilib)	1m 26s
cross (aarch64-unknown-linux-gnu, stable)	1m 12s
cross (powerpc-unknown-linux-gnu, 1.81.0)	1m 24s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 19s
cross (powerpc-unknown-linux-gnu, stable)	1m 17s
test (x86_64-unknown-linux-gnu, 1.81.0)	1m 6s
test (x86_64-unknown-linux-gnu, stable)	1m 3s
	15m 56s

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