Use MaybeUninit in DecInt #4461

Job	Run time
Check --no-default-features (nightly)	58s
Check (stable)	1m 30s
Check selected Tier 3 platforms (nightly)	3m 46s
Check nightly-only targets (nightly)	23s
Check (nightly)	1m 23s
Check (1.63)	1m 39s
Rustfmt	6s
Test rustix_use_experimental_asm (powerpc64le-linux)	44s
Test rustix_use_experimental_asm (s390x-linux)	53s
Test (ubuntu)	42s
Test use-libc (ubuntu)	24s
Test (ubuntu-20.04)	45s
Test use-libc (i686-linux)	39s
Test (i686-linux)	1m 6s
Test use-libc (aarch64-linux)	39s
Test (aarch64-linux)	1m 7s
Test use-libc (powerpc64le-linux)	40s
Test (powerpc64le-linux)	1m 23s
Test use-libc (riscv64-linux)	31s
Test (riscv64-linux)	1m 5s
Test use-libc (s390x-linux)	40s
Test (s390x-linux)	1m 9s
Test use-libc (arm-linux)	41s
Test (arm-linux)	1m 0s
Test (ubuntu-stable)	37s
Test (i686-linux-stable)	1m 1s
Test (aarch64-linux-stable)	1m 32s
Test (riscv64-linux-stable)	58s
Test (s390x-linux-stable)	59s
Test (powerpc64le-linux-stable)	1m 12s
Test (arm-linux-stable)	2m 18s
Test (ubuntu-1.63)	2m 30s
Test (i686-linux-1.63)	2m 59s
Test (aarch64-linux-1.63)	2m 59s
Test (riscv64-linux-1.63)	2m 46s
Test (s390x-linux-1.63)	2m 41s
Test (powerpc64le-linux-1.63)	3m 13s
Test (arm-linux-1.63)	3m 15s
Test (macos-latest)	39s
Test (macos-13)	51s
Test (windows)	48s
Test (windows-2019)	50s
Test (musl)	50s
	56m 51s

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