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_number.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <cstddef>
 #include <cstring>
 #include <typeindex>
 #include "operand.h"
@@ -23,6 +24,8 @@ public:
 
     bool operator==(const Operand& other) const override;
 
+    std::size_t Hash() const override;
+
     template <typename T>
     static LiteralNumber* Create(T value) {
         static_assert(sizeof(T) == 4 || sizeof(T) == 8);