Locale fallback

The Lookup API implements automatic language matching and locale fallback. These mechanisms are available to in-tree uses that call the C++ function intl::Lookup::New

Locale fallback is the process of searching for translated content, locale data, or other resources by "falling back" from more-specific resources to more-general ones following a deterministic pattern.

As a simple example of fallback, a lookup request for a message with the locale en-US falls back to using a message from the locale en if no en-US-specific message was available to the system.

Fallbacks are not always straightforward. As a simple example, en-US falls back to en, but en-IN falls back go en-GB. The fallback relationships are defined in CLDR, and Fuchsia's localization subsystem uses this information to select the correct fallback localization.

Fuchsia's localized message lookup has two levels of fallback:

Language matching. The user is allowed to specify up to 10 preferred locales, in order of preference. This is supported as part of the type fuchsia.intl.Profile.

This setting can be handed over to the Lookup API at Lookup construction time:
```
auto lookup = intl::Lookup::New({"es", "en-US"});
```
This example shows that the user sets a preference to see messages in the locale es, with a fall back to en-US It is expected that the locale information will come from fuchsia.intl.Profile normally at runtime.
Fallback. The lookup library automatically falls back from a more specific locale to a less specific locale even when not explicitly requested by the user. In the prior example:
```
auto lookup = intl::Lookup::New({"es", "en-US"});
```
This example also defines that if en-US is not available, that the locale should fall back to en. This approach allows us to provide a set of most general locales for products that may not need worldwide coverage while still covering the most common locales.

The combination of the two approaches uses the automated rules to find the best available matching locale, but also allows the library caller to specify fine grained sequence of preferred locales to use.

Future work

See Future work section for details on planned but not yet implemented features related to fallback.