implemented simple evaluator for program,

statements and integer literals, together with
tests

test/utils/fixtures.cpp

@@ -1,3 +1,5 @@
+#include "evaluator/evaluator.hpp"
+#include "object/object.hpp"
 #include "utils.hpp"
 
 #include <doctest.h>
@@ -30,4 +32,9 @@ namespace test::utils {
             "parse_program() returned a null pointer"
         );
     }
+
+    void EvalFixture::setup(std::string source) {
+        ParserFixture::setup(source);
+        result = std::unique_ptr<object::object>(eval::eval(program.get()));
+    }
 } // namespace test::utils