Use `MaybeUninit::{assume_init_ref, write}` #1414

Job	Run time
MSRV (ubuntu-latest)	20s
MSRV (windows-latest)	37s
Test (stable)	31s
Test (beta)	36s
Clippy	8s
Test build-std (aarch64-tvos)	2m 22s
Test (nightly)	30s
Test CUDA support	19s
Test build-std (aarch64-tvos-sim)	4m 12s
Test (linux32)	1m 19s
Rustfmt	6s
Test build-std (x86_64-tvos)	6m 18s
Test (macos)	3m 39s
Test (aarch64-macos)	35s
Test (aarch64-ios)	55s
Test (win32)	1m 25s
Test (win64)	2m 23s
Test (mingw32)	1m 23s
Test (mingw64)	1m 32s
Test (windows-2019)	2m 42s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	1m 29s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	3m 8s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	2m 48s
	39m 17s

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