Add prime marks

I felt that some people may need prime marks sometimes (e.g. marking American feet/inches, arcminutes and arcseconds, mathematics).

I would have liked to also have <Multi_key> <apostrophe> <slash> for single prime (in addition to <Multi_key> <slash> <apostrophe>) so that one could enter the prime mark in both directions but it conflicted with the sequence used to make “Ǿ” and “ǿ”. If you have any suggestions on how to fix that, please add it.