Round decimal division of tuple elements.

(These commits address [Cure Code ticket #1974](http://issue.cc/r3/1974).)

When performing tuple division, the floating point representation of decimal divisors may yield surprising results.  For example, the following expression

    >> 1.1.1 / 0.1

can return `9.9.9` or `10.10.10` depending on the host platform.

To address such discrepancies, call `Round_Dec` with the default rounding strategy (halves round up away from zero) before casting the result of tuple element division to a `REBINT`; this should improve cross-platform consistency.

Note that this change means that the following expressions both evaluate to `10.10.10`:

    >> 1.1.1 / 0.1
    >> 1.1.1 / 0.101