Support floating values

A floating value is an "any" value, such as "Any June". Its type would be `Floating<Month, Month>`. "Any June 1st" would be `Floating<Day, Month>`.

These would need:
    - Approximate differences
    - Formatting
    - Parsing
    - Extraction from Fixed values
    - Initialization from components
    - Enumeration
    - Rounding