rand_core: add blanket impl of TryRngCore for RngCore #1021

Job	Run time
Check doc	32s
test (ubuntu-latest, x86_64-unknown-linux-gnu, stable)	51s
test-cross (ubuntu-latest, powerpc-unknown-linux-gnu, stable)	1m 12s
test (macos-latest, x86_64-apple-darwin, stable)	42s
test-ios	22s
test (windows-latest, x86_64-pc-windows-gnu, stable)	2m 45s
test (windows-latest, x86_64-pc-windows-msvc, beta)	1m 31s
test-miri	4m 27s
test (ubuntu-latest, x86_64-unknown-linux-gnu, MSRV, 1.61.0)	1m 45s
test-no-std	15s
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 15s
	17m 16s

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