Draft: Computer (final)

Main purpose: Split responsibility of Money object and provide solution for basic arithmetics with precision.

For backward compatibility all comparisons and arithmetic is done by first available library (GMP > BcMath > native) with precision to 8 decimal place which should be enough for divisions since default currencies have 3 decimals maximum.

Computations are done like this:

```php
$computer = new MoneyComputer(new Calculators\GmpCalculator(/* precision */8));
$computer->add(1, 1); // '2'
$computer->multiply(new Money(100, Currency::get('CZK')), 1.21); // Money(12100)
```