::snap_to() was intended to round a Beats value to the nearest multiple
of another Beats value. It did not do that, but instead rounded down.
Worse, it used Beats::operator/ which in turn uses int_div_round(),
which is incorrect for a situation where we need integer truncation.
The changes fix the actual arithmetic and add 2 variant functions so that the
API includes round down, round up and round to nearest.
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