Develop Sign, Unsign, Nan & Inf for FixedDecimal

[younies]

In many application for units (especially, mixed units), There are a need to represent the numbers as Inf, Neg, Pos ... etc.

Examples:

1. 3 feet, and 11 inches --> could not be 3 feet and -11 inches
2. 5:03:04 --> could not be 5:-03:+04
3. Inf L/100-km
4. ... etc.

Therefore, we need a way to represent Inf, Neg, Nan ... etc in FixedDecimal

Options for expressing mixed units:

1. List of FixedUnsignedDecimal
   1. New type; FixedDecimal contains it as an inner field
   2. FixedDecimal gets a generic parameter
   3. Compositional fixed decimal types, like Signed<T>
2. List of FixedDecimal, take absolute value of each before formatting
3. List of FixedDecimal, show sign when formatting: 3 feet, -11 inches