Implement `DecInt` without `itoa` #4524

Job	Run time
Check selected Tier 3 platforms (nightly)	5m 8s
Check nightly-only targets (nightly)	25s
Check (stable)	6m 1s
Check --no-default-features (nightly)	1m 0s
Check (nightly)	6m 26s
Rustfmt	6s
Check (1.63)	9m 47s
Test rustix_use_experimental_asm (powerpc64le-linux)	1m 0s
Test (ubuntu)	44s
Test rustix_use_experimental_asm (s390x-linux)	1m 5s
Test use-libc (ubuntu)	39s
Test (ubuntu-20.04)	47s
Test use-libc (i686-linux)	52s
Test (i686-linux)	1m 0s
Test use-libc (aarch64-linux)	56s
Test (aarch64-linux)	1m 14s
Test use-libc (powerpc64le-linux)	57s
Test (powerpc64le-linux)	1m 5s
Test use-libc (riscv64-linux)	54s
Test use-libc (s390x-linux)	1m 2s
Test (riscv64-linux)	1m 6s
Test (s390x-linux)	1m 7s
Test use-libc (arm-linux)	1m 2s
Test (arm-linux)	59s
Test (ubuntu-stable)	47s
Test (i686-linux-stable)	53s
Test (aarch64-linux-stable)	1m 4s
Test (riscv64-linux-stable)	1m 1s
Test (s390x-linux-stable)	1m 4s
Test (powerpc64le-linux-stable)	1m 1s
Test (arm-linux-stable)	57s
Test (ubuntu-1.63)	1m 54s
Test (i686-linux-1.63)	2m 10s
Test (aarch64-linux-1.63)	2m 15s
Test (riscv64-linux-1.63)	2m 9s
Test (s390x-linux-1.63)	2m 15s
Test (powerpc64le-linux-1.63)	2m 16s
Test (arm-linux-1.63)	2m 9s
Test (macos-latest)	41s
Test (macos-13)	51s
Test (windows)	54s
Test (windows-2019)	43s
Test (musl)	44s
	1h 11m 10s

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