Make `enumerate` methods accept a callback that throws

[thomasleese]

This updates the enumerateAsArray and enumerateAsDict methods to accept a row callback function that throws an error. If the function does throw an error it will be propogated to the caller of the enumerate method.

This is inspired by https://github.com/swiftcsv/SwiftCSV/pull/139 with the review comments addressed to hopefully get this functionality in to a release.

[DivineDominion]

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[DivineDominion]

Thank you! 🙌