Refactor lookup() and within()

[marselester]

lookup() signature changed:

• expects address value instead of a pointer
• returns Result wrapper to manage memory
• returns null instead of AddressNotFound error to be consistent with Rust implementation
• returns network as part of Result.network

within iterator still returns structs directly, e.g., City/Country, but they're destroyed after it.next() call (iterator owns their memory).

All geolite2 and geoip2 structs no longer have _arena, init() and deinit().
Metadata also dropped them, now Reader owns its memory.