Q.mul_2exp and Q.div_2exp fail with negative exponent #171

Using zarith 1.14, I get:

utop # #require "zarith";;
utop # Q.mul_2exp Q.one (-1);;
Exception: Invalid_argument "Z.shift_left: count argument must be positive".
utop # Q.div_2exp Q.one (-1);;
Exception: Invalid_argument "Z.shift_left: count argument must be positive".

I.e. Q.mul_2exp and Q.div_2exp fail when the exponent is negative with a confusing error message.

There is a workaround by checking whether the exponent is negative and then using div_2exp instead of mul_2exp and vice versa.

The functions should be fixed to work with negative exponents too (or the error message should be more meaningful and the behavior documented).