Add radix string encoding for Uint, BoxedUint #1752

Job	Run time
clippy	23s
build (1.73.0, thumbv7em-none-eabi)	22s
build-benchmarks	55s
careful	1m 7s
build (1.73.0, wasm32-unknown-unknown)	23s
build (stable, thumbv7em-none-eabi)	20s
cross (aarch64-unknown-linux-gnu, stable)	2m 19s
doc	21s
build (stable, wasm32-unknown-unknown)	19s
minimal-versions	20s
cross (powerpc-unknown-linux-gnu, stable)	2m 26s
miri (x86_64-unknown-linux-gnu)	2m 44s
rustfmt	15s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 42s
miri (s390x-unknown-linux-gnu)	2m 52s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 42s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 45s
test (x86_64-unknown-linux-gnu, stable)	1m 42s
	21m 57s

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