Add locale-based currency formatting

Preliminary patch. Depends on #1531058: Include bartfeenstra/cldr to parse Unicode CLDR number patterns.

This patch depends on #1868952: Update bartfeenstra/cldr.

This patch adds:
- Currency locale patterns that are a combination of locale information and a CLDR number pattern.
- Methods to format amounts using a locale pattern: CurrencyLocaleFormat::format() and Currency::format().
- A delegator to load the right pattern based on environment variables. It tries global $language->currency_country_code first, then variable_get('site_default_country'), both in combination with global $language->language. If these fail, then it loads the pattern for en_US (which is predefined and is consistent with Drupal's default language).
- Tests!

That weight property shouldn't be there.

#3: currency_1848030_01.patch queued for re-testing.

#3: currency_1848030_01.patch queued for re-testing.

Yeah, working on this as we speak. Will probably finish it tomorrow as it's already late.

FYI: I'll ad a follow-up issue to expand the list of number patterns for locale based on CLDR data. This means that if we commit this patch now, we can continue with at least three other issues that depend on this API, but not on specific number patterns being available.