k256: simplify internal helper functions in the scalar arithmetic (#917) #58

Job	Run time
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	3m 16s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 55s
cross (aarch64-unknown-linux-gnu, 1.65.0)	3m 54s
cross (aarch64-unknown-linux-gnu, stable)	1m 57s
cross (powerpc-unknown-linux-gnu, 1.65.0)	3m 31s
cross (powerpc-unknown-linux-gnu, stable)	1m 59s
build (1.65.0, thumbv7em-none-eabi)	1m 38s
build (1.65.0, wasm32-unknown-unknown)	1m 40s
build (stable, thumbv7em-none-eabi)	30s
build (stable, wasm32-unknown-unknown)	33s
doc	30s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	3m 36s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	2m 1s
test (x86_64-unknown-linux-gnu, 1.65.0)	3m 57s
test (x86_64-unknown-linux-gnu, stable)	1m 38s
	32m 35s

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