This fixes an endless loop in duration.cc:67.

This fixes an endless loop in duration.cc:67.

The loop occurred when Duration::Duration(Rational r, bool scale) was called
with r.num() >= 2 * r.den(), f.e. the duration of a longa: r.num = 2,
r.den = 1. If k < 0, the left-shifting-operator << returns an undefined value
instead of right-shifting bits. The fix introduces a new function
shift_left() in misc.hh, which behaves different.