Better syntax for external code loading

[IGI-111]

Instead of calling run_external:

impl MyAbi for Contract {
    extern (TargetAbi, {
        // any prelude process or checks
        // ...
        0xABCDEF // block returns target address
    }) fn foobar() -> u64;
}

[IGI-111]

Importantly, the #[storage()] definition of the extern and of the called function should be able to be different (since you may access storage in the prelude but not in the target function). This should be provided for in the syntax.