rand_core: implement reborrow for `UnwrapMut` #1293

Job	Run time
Check doc	23s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	41s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	57s
Check Clippy and rustfmt	8s
test (macos-latest, x86_64-apple-darwin, stable)	1m 3s
test-ios	22s
test (windows-latest, x86_64-pc-windows-gnu, stable)	3m 17s
test-miri	5m 5s
test-no-std	13s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 29s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.63.0)	2m 55s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	53s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	42s
	18m 8s

Provide feedback