Add functions to allocate zeroed `Arc` and `Rc` #603

Job	Run time
Test on i686-unknown-linux-gnu with cross	1m 4s
Test with miri	1m 6s
Test with -Zsanitizer=address	39s
Test Rust 1.34.0 on ubuntu-latest	1m 21s
Test with -Zsanitizer=memory	41s
Test Rust stable on ubuntu-latest	15s
Test with -Zsanitizer=leak	35s
Test Rust beta on ubuntu-latest	15s
Test Rust nightly on ubuntu-latest	20s
Test Rust stable on macos-latest	11s
Test Rust stable on windows-latest	34s
Test Rust stable-x86_64-gnu on windows-latest	40s
Test Rust stable-i686-msvc on windows-latest	47s
Test Rust stable-i686-gnu on windows-latest	47s
	9m 15s

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