Incorrectly stubbed methods

[AndrolGenhald]

I'm not sure how much of this is due to changes between 1.x and master, but I noticed these issues when comparing the stub here against a stub generated using reflection against ext-decimal 1.4.0:

• Decimal::copy doesn't have a $precision parameter
• Decimal::pow has $value instead of $exponent (could matter for named parameters, odd as it is to use named parameters for a single parameter)
• Decimal::round has $rounding instead of $mode (could matter for named parameters)
• Decimal::between is missing from the stub

If all of these need to be changed I can open a pull request.