make rational arithmetic use checked ops

[simonbyrne]

This makes all rational arithmetic operations use checked integer operations to avoid overflow. Along with #8463, this should solve #2960.

[StefanKarpinski]

This is a big improvement. How's the performance? I'm guessing it's not so bad considering all the gcd operations that get done all the time anyway.

[simonbyrne]

It seems to incur a slight penalty of about 5-10% vs master, but still 30% faster than 0.3 (I assume due to #8410).

[StefanKarpinski]

Nice

[simonbyrne]

I think if we really wanted to tweak performance of rationals, one way would be to have an option to not perform the gcd in the constructor when it isn't required, e.g. for Rational(::Int) or after a multiplication or division.

[StefanKarpinski]

That seems like it would be an improvement. I guess that would require having a way of constructing Rational values, bypassing the gcd. Maybe a//b could do the gcd while Rational(a,b) just constructs.