Precision support for Timestamp
Timestamp stores the value in a `datetime64[ns]` internal representation. For most purposes this is fine, but I'd love if I could choose the internal precision, for example to use `datetime64[s]` instead.

When I do timeseries manipulations that span more than a day, I generally do not need sub-second resolution and I could definitely use the extended range instead. But, more conveniently, this would also allow to pretty-print the result without a ton of trailing zeros without setting some arbitrary threshold.

Incidentally, this would remove the limit of ~500 years, but it's more of an ergonomic change than an actual need. For example, when I need to perform direct manipulations of the values in a DatetimeIndex, I often have to scale up to seconds if I want to perform meaningful fitting and avoid numerical instability. When plotting, it's an extra indirection I need to care about.
