Correct binding to NUMBER in Oracle backend.

On platforms where std::numeric_limits<int>::digits10 == 9,
NUMBER(9,0) can be represented losslessly using C++ type int.
Oracle data type NUMBER(precision,scale) is bound to C++ int
if scale is 0 and precision < std::numeric_limits<int>::digits10.
This comparison prevents conversion of NUMBER(9,0) to int, thus it is corrected
using weaker form: precision <= std::numeric_limits<int>::digits10.

Fixes #126
Close #128

(Original commit message amended by Mateusz Loskot)