implemented simple evaluator for program,

statements and integer literals, together with
tests

test/evaluator/integer.cpp

@@ -0,0 +1,20 @@
+#include "doctest.h"
+#include "utils.hpp"
+
+TEST_SUITE("Eval: integers") {
+    TEST_CASE_FIXTURE(
+        test::utils::EvalFixture,
+        "Simple integer expression statements"
+    ) {
+        struct test_struct {
+            std::string input;
+            int expected;
+        };
+        struct test_struct tests[]{{"5", 5}, {"10", 10}};
+
+        for (auto& t : tests) {
+            setup(t.input);
+            test::utils::test_integer_object(result.get(), t.expected);
+        }
+    }
+}