Make `Uint::as_limbs_mut` const #2025

Job	Run time
minimal-versions	41s
build-benchmarks	49s
build (1.83.0, thumbv7em-none-eabi)	1m 52s
careful	1m 41s
clippy	22s
build (1.83.0, wasm32-unknown-unknown)	1m 43s
cross (aarch64-unknown-linux-gnu, stable)	3m 56s
doc	29s
build (stable, thumbv7em-none-eabi)	1m 54s
miri (x86_64-unknown-linux-gnu)	6m 19s
cross (powerpc-unknown-linux-gnu, stable)	5m 35s
build (stable, wasm32-unknown-unknown)	1m 41s
miri (s390x-unknown-linux-gnu)	6m 20s
rustfmt	12s
test (i686-unknown-linux-gnu, 1.83.0, sudo apt update && sudo apt install gcc-multilib)	2m 47s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	2m 41s
test (x86_64-unknown-linux-gnu, 1.83.0)	2m 5s
test (x86_64-unknown-linux-gnu, stable)	2m 6s
	43m 13s

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