Implement timezone calculations in Rust for a performance boost

[ariebovenberg]

Timezone calculations currently use Python's zoneinfo module—even in the Rust implementation of whenever. This leaves quite a lot of performance on the table, since dealing with Python classes comes with quite some overhead. An earlier benchmark indicates timezone switches can be 3-4x faster (#145).

Downside is the amount of work required: basically reimplementing big parts of the zoneinfo module. An interesting option is to reuse code from the Jiff crate as suggested here