operand: Implement operand hashing and use hashed set for declarations

Instead of manually searching each element in the declarations vector,
use an unordered_set to emplace new declarations avoiding repetition.

src/literal_string.cpp

@@ -4,6 +4,7 @@
  * Lesser General Public License version 3 or any later version.
  */
 
+#include <functional>
 #include <string>
 #include "common_types.h"
 #include "literal_string.h"
@@ -36,4 +37,8 @@ bool LiteralString::operator==(const Operand& other) const {
     return false;
 }
 
+std::size_t LiteralString::Hash() const {
+    return Operand::Hash() ^ std::hash<std::string>{}(string);
+}
+
 } // namespace Sirit