Introduce a new conversion function to the libdecnumber library.
This function converts a decNumber to a signed 64-bit integer.
In order to support 64-bit integers (which may have up to 19
decimal digits), the existing "powers of 10" array is expanded
from 10 to 19 entries.

Signed-off-by: Tom Musta <tommusta@gmail.com>
[agraf: fix 32bit host compile]
Signed-off-by: Alexander Graf <agraf@suse.de>