[phobos] Rational for review

[David Simcha]

  I've cleaned up/fixed the bit rot in my Rational library.  It's 
available for review at:  
http://dsource.org/projects/scrapple/browser/trunk/rational/rational.d?rev=785 
.

Known issues:

1.  Due to bugs in BigInt (mainly 4742 
http://d.puremagic.com/issues/show_bug.cgi?id=4742), BigInt 
instantiations don't play nicely with builtin integer instantiations.  
For example, this doesn't work:

auto a = rational(BigInt(1), 2) * rational(8, 7);

2.  I couldn't figure out how to handle the case of user-defined 
fixed-width integer types in the decimal conversion (opCast) function, 
so it just assumes non-builtin integer types are arbitrary precision.  
The biggest problem is lack of a way of introspecting whether the 
integer is fixed or arbitrary width and what its fixed width might be if 
it is, in fact, fixed.  The secondary problem is that I don't know of a 
good algorithm (though I'm sure I could find one if I tried) for 
converting fixed-width integers to floating point numbers in software.  
Arbitrary precision is much easier.

3.  There is a small amount of special case ad-hockery to make things 
work with std.bigint.BigInt.  I consider these bug workarounds.  In 
principle, everything should work with any user defined arbitrary 
precision integer if it overloads everything it's supposed to overload.