Make Big(Int|Float)::power() consistent with IEEE 754 rules

[thekid]

See #5, this makes the method align with how the pow function and the ** operator in PHP 9.0 will behave, albeit using the lang.IllegalArgumentException class instead of the DivisionByZeroError for consistency with the rest of this library's methods.

⚠️ This is a BC break, earlier versions expected 0 as a result of this operation.