libdecnumber: Move common definition under DPD

This patch moves headers with common decimal defintions (range,
exponent, etc.) to libdecnumber common folder.  This is to avoid the
need to duplicate it on bid folder and also to organize headers better.
libdecnumber: Move common debug directive to common header

This patch moves the implementation of decNumberShow to a common header.
libdecnumber: Add initial BID support for _Decimal64

This patch add Binary Integer Decimal encoding support on libdecnumber
for _Decimal64 type.  All libdfp tests now pass, with some exceptions
due ULPs issues (since x86_64 does not have a ULP tests file).