Custom type

Hello,

I've adapted the shopspring decimal.Decimal type for uint256.Int (https://github.com/holiman/uint256).

Even though uint256.Int types can be used as non-pointer types, they are usually used as pointer types, so in TryWrapNumericScanPlan() I'm handling both *uint256.Int and **uint256.Int. I'm wondering however where the proper place to allocate a fresh uint256.Int for the **uint256.Int case would be.

I'm currently doing it in the scan plan Scan() method:

```
func (plan *wrapUint256ScanPlan) Scan(src []byte, dst interface{}) error {
switch dst := dst.(type) {
case *uint256.Int:
return plan.next.Scan(src, (*Uint256)(dst))
case **uint256.Int:
*dst = new(uint256.Int)
return plan.next.Scan(src, (*Uint256)(*dst))
default:
panic("")
}
}
```

This works fine, but I'm wondering if this is really the proper spot.

Thanks for any advice!

Start by reading TryWrapNumericScanPlan() and the shown wrapUint256ScanPlan.Scan() implementation, focusing on handling *uint256.Int and **uint256.Int. Done means determining the appropriate allocation point for **uint256.Int; the payload names no project file or test to run.

Written by the indexing model from the issue text.