change the semantics of `TimeZoneDatabase` from a product type to a sum type

[BurntSushi]

Today, Jiff tries to load as many tzdb's as possible depending on the kind. For example, if for some reason Jiff can find a zoneinfo database, a concatenated database and has a bundled database, then it will look through all three on a tz lookup:

jiff/src/tz/db/mod.rs

Lines 358 to 388 in 94e8270

	pub fn get(&self, name: &str) -> Result<TimeZone, Error> {
	let inner = self.inner.as_deref().ok_or_else(|| {
	if cfg!(feature = "std") {
	err!(
	"failed to find time zone `{name}` since there is no \
	time zone database configured",
	)
	} else {
	err!(
	"failed to find time zone `{name}`, there is no \
	global time zone database configured (and is currently \
	impossible to do so without Jiff's `std` feature \
	enabled, if you need this functionality, please file \
	an issue on Jiff's tracker with your use case)",
	)
	}
	})?;
	if let Some(tz) = inner.zoneinfo.get(name) {
	trace!("found time zone `{name}` in {:?}", inner.zoneinfo);
	return Ok(tz);
	}
	if let Some(tz) = inner.concatenated.get(name) {
	trace!("found time zone `{name}` in {:?}", inner.concatenated);
	return Ok(tz);
	}
	if let Some(tz) = inner.bundled.get(name) {
	trace!("found time zone `{name}` in {:?}", inner.bundled);
	return Ok(tz);
	}
	Err(err!("failed to find time zone `{name}` in time zone database"))
	}

I think I did things this way originally because I was thinking about a bundled database "augmenting" an incomplete zoneinfo database. But this is making less sense now with Jiff supporting Android's concatenated database. Overall, I think this is too much "smarts" for not a lot of gain. And it's somewhat less predictable in terms of its behavior.

In jiff 0.2, I plan to make TimeZoneDatabase represent only a single source of tzdb. And which tzdb is used depends on which is found. And different platforms can prioritize different tzdb sources. For example, on Android, I plan to prioritize the concatenated tzdb so that we aren't always looking for the zoneinfo tzdb unnecessarily.