Don't crash string- and num-ifying Numbers (#55)

[catb0t]

Going to change the return values and add tests as well, so don't merge yet.

closes #55

[catb0t]

The number of places where this computed goto is used in Number is large, I think adding checking code is actually the wrong fix, because such checks are absent from lots of code that doesn't crash

I guess the right fix would be like changing how pointers-to null/nil are handled more fundamentally(?), but I don't know how to do that

[catb0t]

hm, 46 more subs that need this check code added? I really think this is wrong, but I'll write the right fix if you tell me what that is :)

[trizen]

No need to handle this. This would require changing the entire Number class. Since Number objects are immutable by default, the self value is always expected to be a Number object.