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use new arith api (#5) #15

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	43s
cross (armv7-unknown-linux-gnueabihf, 1.81.0)	1m 30s
build (1.81.0, wasm32-unknown-unknown)	44s
doc	50s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 32s
build (stable, thumbv7em-none-eabi)	47s
cross (aarch64-unknown-linux-gnu, 1.81.0)	1m 23s
build (stable, wasm32-unknown-unknown)	35s
cross (aarch64-unknown-linux-gnu, stable)	1m 11s
test (i686-unknown-linux-gnu, 1.81.0, sudo apt update && sudo apt install gcc-multilib)	1m 54s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 43s
test (x86_64-unknown-linux-gnu, 1.81.0)	1m 24s
cross (powerpc-unknown-linux-gnu, 1.81.0)	1m 26s
test (x86_64-unknown-linux-gnu, stable)	1m 29s
cross (powerpc-unknown-linux-gnu, stable)	1m 19s
	18m 30s

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