Previously, they borrowed the buffer, so users had some difficulty to
manage the lifetime of the buffer and Value/Field. For example,
a user structure was unable to contain both the buffer and a Field.
- Field::display_value and DisplayValue::with_unit were introduced.
- The string "centimeter" was changed to "cm" for consistency
with "m", "mm", and "km".
- F-number was changed to be prefixed by "f/" only when with_unit() is
called, though it is actually not a unit.
KAMADA Ken'ichi