Fix FastRational and test it s.t. it is reliable

[Tomaqa]

Currently, FastRational uses signed integer for numerator and unsigned for denominator. This yields conversion issues and mixing such variables is not recommended in C++. I would be probably better to stick to just signed integers (e.g. as in boost::rational) and detect whether they overflow after arithmetic operations.
It can be also worth trying whether using boost::rational directly (if the values fit into 32 bits) would not be more reliable and also more efficient.
Supporting the class with tests would be good.

#712 identifies a bug in constructor caused by incorrect casting.

[blishko]

I believe the only reason for using unsigned type for denumerator is to extend the range of values for which we can use the fast representation.
But maybe it won't make much difference and could simplify the code.

I am against adding a dependency on boost unless there is a clear benefit.