Switch rustflags from `-g` to `-Cdebuginfo=2`
    
    Since rustc doesn't allow `-g` and `-Cdebuginfo` options to mix, we need
    to make sure we're consistent with the option crates may set in their
    own `[profile] debug` settings.  Cargo used to send `-g` for that, but
    switched to `-Cdebuginfo` in version 0.17.0.
    
    Fixes #32.