Adding `zeroize` for `BlockRng` and `BlockRng64` #643

Job	Run time
Check doc	1m 9s
test-ios	35s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	57s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	2m 11s
test (macos-latest, x86_64-apple-darwin, stable)	1m 33s
test-miri	2m 22s
test-no-std	14s
test (windows-latest, x86_64-pc-windows-gnu, stable)	2m 32s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 49s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.60.0)	3m 12s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 39s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 5s
	19m 18s

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