implemented the first version of statement_cache

include/matador/sql/executor.hpp

@@ -12,14 +12,22 @@ struct query_context;
 class query_result_impl;
 class statement;
 
-class executor {
+class dialect_executor_mixin {
 public:
-  virtual ~executor();
-
+  virtual ~dialect_executor_mixin() = default;
   [[nodiscard]] virtual const class dialect& dialect() const = 0;
+};
+
+class prepared_executor : public dialect_executor_mixin {
+public:
+  [[nodiscard]] virtual utils::result<statement, utils::error> prepare(const query_context &ctx) = 0;
+};
+
+class executor : public dialect_executor_mixin {
+public:
   [[nodiscard]] virtual utils::result<size_t, utils::error> execute(const query_context &ctx) const = 0;
   [[nodiscard]] virtual utils::result<std::unique_ptr<query_result_impl>, utils::error> fetch(const query_context &ctx) const = 0;
-  [[nodiscard]] virtual utils::result<statement, utils::error> prepare(const query_context &ctx) const = 0;
+
   [[nodiscard]] virtual std::string str(const query_context &ctx) const = 0;
 };