chore: cleanup zoneinfo_rs crate

[nekevss]

This PR does some general cleanup for the zoneinfo_rs crate. There are no major code changes besides organizational changes and alteration to module paths.

It primarily completes the following:

• Splits apart the types in the types module between rule, zone, and shared.
• Adds some documentation to the general types
• Adds an unstable feature and flags zoneinfo's tzif support

The goal here is generally to clean up this crate for a 0.1.0 release.