Add method RATIONAL to convert a quad-double to a rational.

qd-package.lisp:
o Appropriately shadow and export RATIONAL.
o Need to export WITH-QD-PARTS from QDI.

qd-methods.lisp:
o Define methods for RATIONAL for reals and qd-reals.

qd-complex.lisp:
o Use CL:RATIONAL as appropriate for the CL rational type.