Add coalesce free function and Row method

[philmitchell]

As discussed with @groue, added coalesce methods as a best-effort alternative in cases where nil-coalescing fails. (I did investigate addressing the failure directly, but given the double optionals that represent null values (.some(.none)), it seems unlikely to bear fruit.)

Pull Request Checklist

• CONTRIBUTING: You have read https://github.com/groue/GRDB.swift/blob/master/CONTRIBUTING.md
• BRANCH: This pull request is submitted against the development branch.
• DOCUMENTATION: Inline documentation has been updated.
• DOCUMENTATION: README.md or another dedicated guide has been updated.
• TESTS: Changes are tested.
• TESTS: The make smokeTest terminal command runs without failure.

[groue]

Hello @philmitchell, I'm glad to hear about you :) Please give me a couple of days before I can review the pull request.