I would expect BigDecimal#divmod to return type [Int, BigDecimal] (like Integer#divmod, Float#divmod)

`Integer#divmod` returns type `[Integer, Integer]`
`Float#divmod` returns type `[Integer, Float]`
`BigDecimal#divmod` returns type `[BigDecimal, BigDecimal]`

Which feels surprising.

```
1.divmod(2)
=> [0, 1]
(1.0).divmod(2)
=> [0, 1.0]
BigDecimal(1).divmod(2)
=> [0.0, 0.1e1]
```

(thank you BigDecimal maintainers, you are wonderful)