Consolidate add and offset functions

[Felixoid]

Is your feature request related to a problem? Please describe.
As I see and discuss with @dzhdanov, add and offset are pretty much the same functions with different descriptions and small implementation details.

Describe the solution you'd like
One function should be an alias to another with only one implementation

Describe alternatives you've considered
The alternative would be to leave it as it is, but now it doesn't look good, even from the documentation side: add vs offset

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[Felixoid]

Is it still the case?

[deniszh]

I think so. Pinned.