codeprinters: fix Rational's printing for code (as fractions of doubles)

S(4)/3 * x -> (4.0/3.0)*x for ccode, not 4*x/3

See also:
https://github.com/sympy/sympy/pull/1763

@jrioux:
> As to the printing of Rational(2, 3)*x, I agree that it should use doubles, just like printing Rational(2, 3) on its own does; looks like a dedicated _print_Mul should be implemented for CCodePrinter (maybe even for the parent class CodePrinter).

That's done.  Looks a bit ugly: _print_Mul() method copy-pasted from StrPrinter with minimal changes.  This code was omitted:
```
            elif item.is_Rational and item is not S.Infinity:
                if item.p != 1:
                    a.append(Rational(item.p))
                if item.q != 1:
                    b.append(Rational(item.q))
```
(n.b: why Rationals are here, not Integers?)

Second.  C99 says:
> An unsuffixed floating constant has type double. 

May be we should change _print_Rational?