move to enketo-oc

[MartijnR]

• move functions
• move tests

[MartijnR]

This would mean installing the xpath evaluator differently in https://github.com/OpenClinica/xpath-evaluator-app. Not sure if we can do a clean install from a specific package in a monorepo, or if it's worth publishing the oc-extended xpath-evaluator on npm.

May be best to leave this as is until it causes a problem.