libdfp: Remove duplicated function call in truncd
libdecnumber: Add initial BID support for _Decimal32

This patch add Binary Integer Decimal encoding support on libdecnumber
for _Decimal32 type.  All libdfp tests now pass, with some exceptions
due ULPs issues (since x86_64 does not have a ULP tests file).
libdfp: quantumd[32|64|128] refactor

This patch changes quantumd implementation to use decnumber structures
directly instead of getexpd interface to set the exponent from input.
It also refactor quantumd[64|128] to avoid the include/macro mess and
using a more straight implementation.