Round decimal division of tuple elements.

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, `round(3)` before casting the result of
tuple element division to a `REBINT`; this should improve cross-platform
consistency.
Use R3's rounding implementation instead of round(3).

This should ensure greater portability as `round(3)` was standardised as
part of C99.
Merge pull request #104 from 0branch/tuple-div-rounding

Round decimal division of tuple elements. This submission fixes the main problem, but there could be a few edge case issues remaining as pointed out by Ladislav.