Add `rand_core::InfallibleRng` marker trait #732

Job	Run time
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 30s
test-ios	23s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	1m 10s
Check doc	1m 4s
test (macos-latest, x86_64-apple-darwin, stable)	2m 7s
test-miri	3m 28s
test (windows-latest, x86_64-pc-windows-gnu, stable)	1m 55s
test-no-std	15s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 52s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	2m 0s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 42s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 29s
	18m 55s

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