Add a toNumericString method

[BackEndTea]

This method guarantees the output is numeric (or an exception is thrown).

Our code base still has some 'old' code where numeric strings are expected, and calculations are done with them. By having a method which guarantees the output is numeric, we can continue doing math with that.

[BenMorel]

Hi, I fail to see the value in this PR:

• for BigDecimal, you haven't provided an example of a BigDecimal value that would fail the is_numeric() test, and have fallen back to artifically creating an invalid object with reflection to test this
• for BigRational, it's already quite easy to test if the rational number can be converted to a numeric string: just convert toBigDecimal() an catch a potential exception

[BenMorel]

Closing due to lack of feedback.