Rust

Allow casting between slices of ZSTs and slices of non-ZSTs in all ca… #549

Job	Run time
Test on i686-unknown-linux-gnu with cross	1m 6s
Test with miri	48s
Test with -Zsanitizer=address	37s
Test Rust 1.34.0 on ubuntu-latest	32s
Test with -Zsanitizer=memory	39s
Test Rust stable on ubuntu-latest	12s
Test with -Zsanitizer=leak	34s
Test Rust beta on ubuntu-latest	12s
Test Rust nightly on ubuntu-latest	17s
Test Rust stable on macos-latest	17s
Test Rust stable on windows-latest	17s
Test Rust stable-x86_64-gnu on windows-latest	16s
Test Rust stable-i686-msvc on windows-latest	19s
Test Rust stable-i686-gnu on windows-latest	18s
	6m 24s

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