Add `BoxedUint::conditional_select` #867

Job	Run time
clippy	22s
build (1.70.0, thumbv7em-none-eabi)	47s
build-benchmarks	48s
build (1.70.0, wasm32-unknown-unknown)	43s
cross (aarch64-unknown-linux-gnu, stable)	2m 29s
build (stable, thumbv7em-none-eabi)	45s
doc	22s
cross (powerpc-unknown-linux-gnu, stable)	2m 49s
build (stable, wasm32-unknown-unknown)	40s
dudect	13s
rustfmt	13s
test (i686-unknown-linux-gnu, 1.70.0, sudo apt update && sudo apt install gcc-multilib)	1m 56s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 42s
test (x86_64-unknown-linux-gnu, 1.70.0)	1m 25s
test (x86_64-unknown-linux-gnu, stable)	1m 27s
	16m 41s

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