Version   Changes
--------  --------------------------------------------------------------------
0.5.0     Added pre-defined quantities (with corresponding features):
          - Area
          - Volume
          - Speed
          - Acceleration
          - Force
          - Energy
          - Power
          - Frequency
          - Datathroughput

0.4.2     Changed order of generated code in macro 'quantity' (fixes broken
          doc).

0.4.0     Added fn Unit::from_symbol and fn Unit::from_scale.
          Added fn Quantity::unit_from_symbol and
          fn Quantity::unit_from_scale.
          Removed struct Qty<U> and its generic impls of some std traits.
          Removed type Unitless by implementing trait Quantity directly for
          AmountT.
          Added trait HasRefUnit.
          Enhanced attribute macro 'quantity' to derive a quantity as product
          or quotient of quantities.

0.3.0     Proc-macro 'quantity' – attribute 'unit':
           * Allow int literal as 'scale' param.
           * Added optional 'doc' param.
          Added modules with predefined quantities Mass, Length, Duration and
          DataVolume.

0.2.1     Fixed missing doc attributes in code generated by proc-macro
          'quantity'.
          Fixed cfg attributes to use fpdec.

0.2.0     Replaced macro 'define-qty' by proc_macro_attribute 'quantity'.
          Added impls of traits 'PartialEq', 'Eq' and 'PartialOrd'.
          Added mod prelude.
          Renamed NON_UNIT to ONE.
          Added fn Unit::iter and fn Quantity::iter_units.
          Enhanced documentation.

0.1.0     First public version.
