Implement `DecInt` without `itoa` #4526

Job	Run time
Rustfmt	3s
Check --no-default-features (nightly)	55s
Check nightly-only targets (nightly)	23s
Check (stable)	6m 26s
Check (nightly)	6m 7s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 6s
Test rustix_use_experimental_asm (s390x-linux)	59s
Check (1.63)	9m 10s
Test (ubuntu)	42s
Test use-libc (ubuntu)	44s
Test (ubuntu-20.04)	44s
Test use-libc (i686-linux)	59s
Test (i686-linux)	1m 0s
Check selected Tier 3 platforms (nightly)	4m 56s
Test use-libc (aarch64-linux)	1m 49s
Test (aarch64-linux)	1m 14s
Test use-libc (powerpc64le-linux)	52s
Test (powerpc64le-linux)	1m 0s
Test use-libc (riscv64-linux)	52s
Test (riscv64-linux)	1m 14s
Test use-libc (s390x-linux)	2m 0s
Test (s390x-linux)	1m 11s
Test use-libc (arm-linux)	1m 1s
Test (arm-linux)	1m 8s
Test (ubuntu-stable)	42s
Test (i686-linux-stable)	1m 2s
Test (aarch64-linux-stable)	56s
Test (riscv64-linux-stable)	1m 3s
Test (s390x-linux-stable)	59s
Test (powerpc64le-linux-stable)	1m 6s
Test (arm-linux-stable)	1m 13s
Test (ubuntu-1.63)	1m 51s
Test (i686-linux-1.63)	2m 20s
Test (aarch64-linux-1.63)	2m 19s
Test (riscv64-linux-1.63)	2m 21s
Test (s390x-linux-1.63)	2m 9s
Test (powerpc64le-linux-1.63)	2m 19s
Test (arm-linux-1.63)	2m 15s
Test (macos-latest)	29s
Test (macos-13)	53s
Test (windows)	42s
Test (windows-2019)	48s
Test (musl)	39s
	1h 12m 41s

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