*: use transparent comparison for std::{map,set} with std::string keys

This avoids many std::string temporaries for lookups.

src/sticker/Database.cxx

@@ -129,7 +129,7 @@ StickerDatabase::LoadValue(const char *type, const char *uri, const char *name)
 }
 
 void
-StickerDatabase::ListValues(std::map<std::string, std::string> &table,
+StickerDatabase::ListValues(std::map<std::string, std::string, std::less<>> &table,
 			    const char *type, const char *uri)
 {
 	sqlite3_stmt *const s = stmt[STICKER_SQL_LIST];

src/sticker/Database.hxx

@@ -127,7 +127,7 @@ public:
 		  void *user_data);
 
 private:
-	void ListValues(std::map<std::string, std::string> &table,
+	void ListValues(std::map<std::string, std::string, std::less<>> &table,
 			const char *type, const char *uri);
 
 	bool UpdateValue(const char *type, const char *uri,

src/sticker/Sticker.hxx

@@ -8,7 +8,7 @@
 #include <string>
 
 struct Sticker {
-	std::map<std::string, std::string> table;
+	std::map<std::string, std::string, std::less<>> table;
 };
 
 #endif