Allow to override native methods / add hooks for debugging

[peq]

For example to hook into SetUnitX and catch out of bounds errors.

[Cokemonkey11]

Would it be more, or less, healthy to shadow natives like SetUnitX with a custom implementation?

@shadow_native
function SetUnitX(unit u, real x)
    print("SetUnitX called with " + x.toString())
    super // Call this what you want, but no need to parameterize I guess since args can be inferred

hook is one of the "dirty" vjass features in my mind.