Add ion_decimal_is_signed to provide consistent negative zero behavior

[tgregg]

ion_decimal_is_negative currently returns a different value for negative zero depending on whether it is represented with a decQuad or decNumber under the hood. In order to fix this, we should add ion_decimal_is_signed, which would call decQuadIsSigned for decQuad and decNumberIsNegative for decNumber. Then we should document ion_decimal_is_negative's subtly different behavior and deprecate it.

For more information, see #249