unitd.Num -> unitd.Measure
Precise Measurements

Measurements now track precision. Operations between measurements
will result in a measurement with the lower of the two precisions. The
'val()' method truncates the number at the desired precision. Calculated
measurements with 'infinite' precision are available behind the 'raw()'
method.

Measurements created without an explicit precision are presumed to have
'infinite' precision. This may change in the future.

NOTE: The ECMAScript spec only requires between 1 and 21 digits of
precision, although some VMs may support more, we cap at 21 for
consistency.