strs: Storage class consistency

fwd provides forward declarations of using `class`, but these are defined
in strs.hpp using struct. Only a problem if you both use an explicit `struct`
and use link time optimization - the warning about violating ODR is emitted
during link time with -flto, there is no warning in an isolated compilation
unit.

I do not fully understand why yet.
See the following "never-expire" pastebin of a diff that attempts to create
an upper bound for a a more isolated test case, and fails to trigger the ODR
warning:
https://0bin.net/paste/jDz63gch#DM2x09aMX+iaIAaFWI-WmeSyyKto5F+vZzSniHoHfI6

1 files changed, 7 insertions, 7 deletions

diff --git a/src/mmo/fwd.hpp b/src/mmo/fwd.hpp
index d98bd33..6a3ece5 100644
--- a/src/mmo/fwd.hpp
+++ b/src/mmo/fwd.hpp
@@ -46,14 +46,14 @@ class BlockId;
 class GmLevel;
 class ClientVersion;
 
-class AccountName;
-class AccountPass;
-class AccountCrypt;
-class AccountEmail;
-class ServerName;
-class PartyName;
+struct AccountName;
+struct AccountPass;
+struct AccountCrypt;
+struct AccountEmail;
+struct ServerName;
+struct PartyName;
+struct VarName;
 class QuestId;
-class VarName;
 class MapName;
 class CharName;