Add `decimal` conversion for `DECIMAL`

Something like this at least and/or conversion operators would be very useful.

``` C#
public decimal ToDecimal()
{
    return new decimal(
        (int)Anonymous2.Anonymous.Lo32,
        (int)Anonymous2.Anonymous.Mid32,
        (int)Hi32,
        Anonymous1.Anonymous.sign == 0x80,
        Anonymous1.Anonymous.scale);
}
```