Use minimum necessary division precision in BigDecimal_DoDivmod

[tompng]

Improve performance of div and divmod in some case.

BigDecimal('7'*100000).div(BigDecimal('3'*99999)) #=> 23
# processing time: 0.566657s → 0.001474s

In the above example, div needs to calculate division with 2-digits precision. But it was internally calculating with 200000-digits precision.

This pull request also fixes the cause of this bug (found while adding test)

BigDecimal('3e20').div(BigDecimal('-7e-19'))
# => -428571428571428571428571428571000000000 (bigdecimal-3.2.1)
# => -428571428571428571428571428571428571429 (expected result)

but this bug was fixed by a side effect of #377