Improve SmallRng initialization performance #989

Job	Run time
Check doc	30s
Check doc	34s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 25s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 11s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	52s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	1m 0s
test (macos-latest, x86_64-apple-darwin, stable)	51s
test (macos-latest, x86_64-apple-darwin, stable)	58s
test-ios	25s
test-ios	19s
test-miri	3m 16s
test-miri	4m 28s
test (windows-latest, x86_64-pc-windows-gnu, stable)	1m 48s
test (windows-latest, x86_64-pc-windows-gnu, stable)	3m 2s
test-no-std	11s
test-no-std	12s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 31s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 44s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	2m 47s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	1m 47s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 20s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 55s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 12s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 16s
	34m 34s

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