Feature: parttime subtraction to produce a parital difftime

To provide a more familiar interface for creating difftimes, parttime should provide:

```
`-`(, ) ->
```

In all cases, a partial difftime should be the widest resulting range from two partial difftimes based on the upper and lower imputed values. This includes missing-in-the-middle partial datetimes.

No files or tests are named. Start by locating the partial_time subtraction and partial_difftime implementations in the parttime R package, then trace how missing-in-the-middle partial datetimes are represented. Done means subtraction accepts two partial times and returns the widest partial difftime range based on upper and lower imputed values.

Written by the indexing model from the issue text.