rand_core: introduce an UnwrapMut wrapper #1284

Job	Run time
Check doc	21s
Check Clippy and rustfmt	8s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	52s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	41s
test-miri	5m 1s
test-ios	21s
test (macos-latest, x86_64-apple-darwin, stable)	41s
test-no-std	13s
test (windows-latest, x86_64-pc-windows-gnu, stable)	3m 55s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 24s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.63.0)	2m 47s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	56s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	43s
	18m 3s

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