Convert the country and locale resolvers to return value objects

Right now the country and locale resolvers return strings. Instead, they should return value objects, which can later be expanded with caching metadata.

Rough code:

// in src/Country.php
final class Country {
    protected $countryCode;

    public function __construct($countryCode) {
        $this->countryCode = strtoupper($countryCode);
    }

   public function getCountryCode() {
      return $this->countryCode;
   }

    public function __toString() {
        return $this->countryCode;
    }
}

// in src/Locale.php
final class Locale {
    protected $localeCode;

    public function __construct($localeCode) {
        $this->localeCode = strtoupper($localeCode);
    }

   public function getLocaleCode() {
      return $this->localeCode;
   }

    public function __toString() {
        return $this->localeCode;
    }
}

No interfaces, final classes, no setters. Cause value objects.

Steps:
1) Create the classes and ensure proper docblocks
2) Modify the resolver interfaces (CountryResolverInterface, LocaleResolverInterface) to document the new return values
3) Update the DefaultCountryResolver and DefaultLocaleResolver to wrap the values they return in the new objects.