Turn on sanitizing

[frewsxcv]

rust-lang/rust#38699

[frewsxcv]

https://users.rust-lang.org/t/howto-sanitize-your-rust-code/9378

[frewsxcv]

sancov is on

afl.rs/src/bin/cargo-afl.rs

Line 148 in 2cf8544

-C passes=sancov \

[frewsxcv]

actually if the user attempts to turn these on with RUSTFLAGS, it'll overwrite cargo-afl's RUSTFLAGS

[frewsxcv]

Done in https://github.com/rust-fuzz/afl.rs/pull/137/files