flower: Add boolean return value to 'Rational' class.

flower: Add boolean return value to 'Rational' class.

This removes many clang warnings of the type

  implicit conversion turns floating-point number into integer:
  'double' to 'bool' [-Wfloat-conversion]

[dak, 2019-12-29 11:23:56]

Frankly, I am disconcerted that the original code runs through a float
conversion in the first place.  This is not as much about removing a warning
rather than fixing bad code since the warning had a point.

https://codereview.appspot.com/581400043/diff/577250043/flower/rational.cc
File flower/rational.cc (right):

https://codereview.appspot.com/581400043/diff/577250043/flower/rational.cc#ne...
flower/rational.cc:50: else if (sign_ == -1 || sign_ == 1
That's unreadable and "assert (false)" is not a useful assertion to print out. 
Instead this should not even contain an else if branch (which is entirely
pointless if the if branch ends in return).

The whole function body could just be
   assert (sign_ >= -2 && sign <= 2);
   return sign_ != 0;

but I don't think the assertion is even warranted.  Why check for stuff that the
function doesn't need to work?

[lemzwerg, 2019-12-29 11:52:25]

simplified

[lemzwerg, 2019-12-29 11:53:52]

Thanks for the review.  I agree with all your points.