Add `TryRngCore` and `TryCryptoRng` traits #784

Job	Run time
test-ios	31s
Check doc	1m 6s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	54s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	2m 2s
test (macos-latest, x86_64-apple-darwin, stable)	1m 23s
test-miri	3m 39s
test-no-std	12s
test (windows-latest, x86_64-pc-windows-gnu, stable)	2m 51s
test (windows-latest, x86_64-pc-windows-msvc, beta)	2m 5s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	2m 38s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	2m 4s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	1m 25s
	20m 50s

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