Support zeroable and pod traits for fieldless enums #503

Job	Run time
Test on i686-unknown-linux-gnu with cross	1m 11s
Test with -Zsanitizer=address	52s
Test with miri	1m 6s
Test with -Zsanitizer=memory	48s
Test Rust 1.34.0 on ubuntu-latest	1m 35s
Test with -Zsanitizer=leak	44s
Test Rust stable on ubuntu-latest	16s
Test Rust beta on ubuntu-latest	13s
Test Rust nightly on ubuntu-latest	28s
Test Rust stable on macos-latest	17s
Test Rust stable on windows-latest	28s
Test Rust stable-x86_64-gnu on windows-latest	33s
Test Rust stable-i686-msvc on windows-latest	37s
Test Rust stable-i686-gnu on windows-latest	41s
	9m 49s

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