Add math methods

[vudaltsov]

• abs(): self
• negated(): self
• compareTo(self): -1|0|1
• isEqualTo(self): bool
• isLessThan(self): bool
• isLessThanOrEqualTo(self): bool
• isGreaterThan(self): bool
• isGreaterThanOrEqualTo(self): bool
• isZero(self): bool
• isNegative(self): bool
• isNegativeOrZero(self): bool
• isPositive(self): bool
• isPositiveOrZero(self): bool