Add `TryRngCore` and `TryCryptoRng` traits #813

Job	Run time
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 25s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	49s
Check doc	1m 3s
test-ios	19s
test (macos-latest, x86_64-apple-darwin, stable)	44s
test (windows-latest, x86_64-pc-windows-gnu, stable)	1m 28s
test-miri	3m 26s
test-no-std	12s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 37s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	2m 44s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	1m 41s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 29s
	16m 57s

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