Prevent call site environment from affecting assume!'s behavior

[yvt]

Paths generated by macros are resolved in the call site's context. This means unprefixed paths like cfg!can refer to whichever the item imported in the call site's environment, not the one defined by the standard library. This PR rewrites all such paths to fix the issue.

/// Rogue macro
macro_rules! cfg {
    ($($tt:tt)*) => {
        return
    };
}

/// Rogue "crate"
mod core {}

#[test]
#[should_panic]
#[cfg(debug_assertions)]
fn should_not_affected_by_call_site_environment() {
    assume!(unsafe: @unreachable);
}

[NicholasGorski]

Really appreciate you making this better, thank you!