Refactor GeoIP2 structs for ergonomic access

Replace BTreeMap-based names with typed Names struct using Option<&str>
fields for each language. Make nested struct fields non-optional with
Default, while keeping leaf values as Option<T> to preserve semantics.

This eliminates nested Option unwrapping:
- Old: city.city.as_ref().and_then(|c| c.names.english)
- New: city.city.names.english

Add is_empty() methods using *self == Self::default() pattern so they
don't need updating when fields are added.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: oschwald

CHANGELOG.md

@@ -68,6 +68,22 @@
   added for user input errors (e.g., looking up an IPv6 address in an IPv4-only
   database). Previously this returned `InvalidDatabase`, which incorrectly
   suggested the database was corrupted.
+- **BREAKING CHANGE:** The `names` fields in GeoIP2 structs now use a `Names`
+  struct instead of `BTreeMap<&str, &str>`. This improves performance (no map
+  allocation) and ergonomics. Each language field is `Option<&str>`:
+  - Old: `city.names.as_ref().and_then(|n| n.get("en"))`
+  - New: `city.city.names.english`
+  - Supported languages: `german`, `english`, `spanish`, `french`, `japanese`,
+    `brazilian_portuguese`, `russian`, `simplified_chinese`
+- **BREAKING CHANGE:** Nested struct fields in GeoIP2 record types (`City`,
+  `Country`, `Enterprise`) are now non-optional with `Default`. This simplifies
+  access patterns by removing nested Option unwrapping:
+  - Old: `city.city.as_ref().and_then(|c| c.names.english)`
+  - New: `city.city.names.english`
+  - Old: `city.subdivisions.as_ref().map(|v| v.iter())`
+  - New: `city.subdivisions.iter()` (empty Vec if not present)
+  - Leaf values (strings, numbers, bools) remain `Option<T>` to preserve
+    the distinction between "not present" and "present but empty"
 - Error messages now include byte offsets when available, making it easier to
   debug malformed databases. The `#[non_exhaustive]` attribute is added to
   `MaxMindDbError` to allow future additions without breaking changes.

examples/within.rs

@@ -25,12 +25,9 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
             continue; // Skip networks without data
         };
 
-        let continent = info.continent.and_then(|c| c.code).unwrap_or("");
-        let country = info.country.and_then(|c| c.iso_code).unwrap_or("");
-        let city = match info.city.and_then(|c| c.names) {
-            Some(names) => names.get("en").copied().unwrap_or(""),
-            None => "",
-        };
+        let continent = info.continent.code.unwrap_or("");
+        let country = info.country.iso_code.unwrap_or("");
+        let city = info.city.names.english.unwrap_or("");
         if !city.is_empty() {
             println!("{} {}-{}-{}", network, continent, country, city);
         } else if !country.is_empty() {