Reduce size of the atan partition and node tables by half.

qd-const.lisp:
o Update tables and generator to use tables with only half as many
  entries as before. This is because atan-qd/taylor never needs to
  compute directly for x > 1.

qd-fun.lisp:
o Use table length for binary search, so we're independent of other
  constants.
o Remove FIXME.