refactor(api): remove casting from timestamp to integer and integer to timestamp

[cpcloud]

These two operations differ in our backends and IMO are confusing foot guns that make it hard to write portable Ibis code due to differing support for different units across backends. It also doesn't round trip, even within the same backend.

For the timestamp -> integer case, we can make an epoch(unit) API on TimestampValue.

For the integer -> timestamp case, we have the IntegerValue.to_timestamp method which allows specifying units. For that case I think we should also make the units required and not default to seconds.

[ncclementi]

For the timestamp -> integer case, we can make an epoch(unit) API on TimestampValue.

@jcrist you had some comments/opions about this part while we were on triage.

For the integer -> timestamp case, we have the IntegerValue.to_timestamp method which allows specifying units. For that case I think we should also make the units required and not default to seconds.

We can do this easily in the new IntergerValue.as_timestamp() API that we just changed. (PR coming up).

[cpcloud]

I think you mean Integer value in the last bit. IntervalValue.as_timestamp() isn't a meaningful operation in general.

[ncclementi]

Yup, you are right!

[ncclementi]

@jcrist I lost track of the other PR that was tackling part one, and I can't seem to find it, can you reference it here and close this one if it has been merged.

[jcrist]

For the timestamp -> integer case, we can make an epoch(unit) API on TimestampValue.

This was resolved by #9856