Fix Call.value in init()

[nikita-fuchs]

I was puzzled to find out today that Call.value is always 0 inside the init() function, regardless of whether some amount was sent in the tx or not. The current workaround seems to be using Contract.balance, as in that context it represents the same amount (at least for the cases I could think of so far). If we consider this to be formally true, I guess a tiny compiler trick should make Call.value work easily?