Build Zinc with Cargo

Not finished, but I'm just working through it now. You can build the zinc rlib with

    cargo build --target thumbv7em-linux-eabi

I've also removed the macro_ioreg crate and merged into the single ioreg crate as part of this. Next steps are doing the flexible target specification now that is has landed and also making changes to include all the static libs in the one go.
The issue with interrupt handlers is that Rust doesn't do weak/optional functions very well. The implementation of interrupt handlers is app specific (as the app chooses to enable them).
Happy to take suggestions for this, should Zinc provide a generic handle that passes the interrupt to an event handler in the main thread?