Improve addition/substraction with units

I think it could be really useful if the currently implemented unit handling (such as '+1 Monday') would be improved in a way that I could retrieve things like:
- the next/previous Monday only if the specified date is not a Monday
- the start of the next/previous week only if the specified date is not the beginning of a week (would require adding first_weekday from glibc locales)
- same for months and years..

Start by reading the existing unit-handling implementation behind expressions such as '+1 Monday' and identify how dates, weeks, months, and years are currently adjusted. Clarify the expected next/previous behavior for dates already on boundaries, including first_weekday from glibc locales. Done means the specified cases work consistently and are covered by tests.

Written by the indexing model from the issue text.