Rust

revert to 2018 edition so that rust 1.34 will allow us to build the c… #631

Job	Run time
Test with miri	1m 8s
Test with -Zsanitizer=address	38s
Test on i686-unknown-linux-gnu with cross	1m 3s
Test with -Zsanitizer=memory	39s
Test Rust 1.34.0 on ubuntu-latest	1m 6s
Test Rust stable on ubuntu-latest	14s
Test Rust beta on ubuntu-latest	13s
Test Rust nightly on ubuntu-latest	20s
Test Rust stable on macos-latest	23s
Test Rust stable on windows-latest	36s
Test Rust stable-x86_64-gnu on windows-latest	42s
Test Rust stable-i686-msvc on windows-latest	38s
Test Rust stable-i686-gnu on windows-latest	36s
	8m 16s

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