rand_core: implement reborrow for `UnwrapMut` (#1595) #1312

Job	Run time
Check doc	25s
Check Clippy and rustfmt	17s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 4s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	44s
test-ios	20s
test-miri	4m 58s
test-no-std	13s
test (macos-latest, x86_64-apple-darwin, stable)	37s
test (windows-latest, x86_64-pc-windows-gnu, stable)	1m 28s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 26s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.63.0)	2m 59s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 33s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	43s
	16m 47s

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