Check for overflow in Coin.Multiply and return error

[ethanfrey]

Update In the end, we just add a check to Multiply to see if it will overflow. If it will overflow, return an error instead. (Using normal int64 math or big to check it).

Then we can guarantee no overflows can happen

Is your feature request related to a problem? Please describe.

This came up in a PR review comment #322 (comment)

Describe the solution you'd like

Never overflow when calculating with Coins

Describe alternatives you've considered

We can use big, or we can just limit the ranges to make sure we never overflow int64. I think the second case is quite easy, as total token supply should generally fit in int32 and int64 was for super-safety already.

But let's discuss if this is needed and how urgent

[ethanfrey]

My proposal:

Limit each account to store 2^32 in Whole (MaxInt32).
Limit any multiplier to 2^31 (fee dist)
We can never have an overflow.

I think this is allows lots of math using the normal ints and restricting an account to 4 billion tokens shouldn't be a big problem (seriously, even if you had more, please put them behind multiple private keys)