[Feature Request] `AddAssign`/`SubAssign` implementation to `chrono::Duration`

[eval-exec]

We have AddAssign/SubAssign implementation for DateTime/Date this PR: #698

And I think it would be more convinient if there are AddAssign/SubAssign implementation for chrono::Duration

[djc]

We currently maintain compatibility with the time::Duration type depending on features (from time 0.1, which is no longer being maintained), so I think adding this today would be a bit of a compatibility hazard.

[eval-exec]

Thank you 💟

[pitdicker]

Added in #1327.