[WIP] Add important numerical constants

[castarco]

A list of important numerical constants.

Interfaces

• [X] Zero
• [X] One
• [X] PI
• [X] E
• [X] Euler - Mascheroni Constant
• [X] Golden Ratio
• [ ] Silver Ratio
• [X] Light Speed

Arbitrary precision (via $scale parameter)

• [ ] PI
• [X] E
• [ ] Euler - Mascheroni Constant
• [ ] Golden Ratio
• [ ] Silver Ratio

Integration with Decimal

• [X] Use the Zero constant instead of creating new zero objects inside Decimal methods.
• [X] Use the One constant instead of creating new one objects inside Decimal methods.

The non-phisical numerical constants must be associated to a method with the following signature:

public static function ConstantName($scale);

This methods will allow to specify how many digits we want for every one of the supported numerical constants.

[agungsantoso]

Hi @castarco, I create PR to add silver ratio.