Add `TryRngCore` and `TryCryptoRng` traits #783

Job	Run time	Billable time
test-ios	30s	0s
Check doc	1m 5s	0s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 30s	0s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	38s	0s
test-miri	3m 44s	0s
test (macos-latest, x86_64-apple-darwin, stable)	5m 23s	0s
test-no-std	14s	0s
test (windows-latest, x86_64-pc-windows-gnu, stable)	2m 21s	0s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 30s	0s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	2m 12s	0s
test (ubuntu-latest, sudo apt-get update ; sudo apt install gcc-multilib, i686-unknown-linux-gnu,...	56s	0s
test (ubuntu-latest, x86_64-unknown-linux-gnu, nightly, minimal_versions)	46s	0s
	20m 49s	0s

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