fdo#81356: use boost::rational internally in Fraction

Change-Id: I6f40eafee7652209395bd471e3508fe3a3d19d73
Reviewed-on: https://gerrit.libreoffice.org/12085
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>