add --careful with rustc flags that cargo careful uses

[0xalpharush]

Something like RUSTFLAGS="-Zsanitizer=address -Cdebug-assertions=on -Zextra-const-ub-checks -Zstrict-init-checks" cargo +nightly [...]

cargo-fuzz recently did this rust-fuzz/cargo-fuzz#339. There is a conflict between instrumenting coverage and -Z build-std (rust-lang/wg-cargo-std-aware#63)

[smoelius]

I'm a little wary of option creep, but I suppose this is fine.

Want to submit a PR?