Refactor: Remove integration tests and organize into modules

This commit refactors the test suite by removing the monolithic
`integration_test.rs` file.
The tests have been categorized and moved into dedicated modules within
the `tests/` directory:

- `tests/parser.rs`: Contains tests for parsing various literal values.
- `tests/destructuring.rs`: Houses tests related to destructuring
  syntax.
- `tests/closures.rs`: Includes tests for closure behavior and related
  optimizations.
- `tests/optimizer.rs`: Contains tests specifically targeting optimizer
  logic and regressions.
- `tests/records.rs`: Holds tests for record syntax and associated
  optimizations.
- `tests/rtl.rs`: Contains tests for runtime library operators and
  functions.
- `tests/pipeline.rs`: Tests for pipeline functionality.
- `tests/macros.rs`: Tests for macro expansion and related issues.
- `tests/error_recovery.rs`: Includes tests for compiler error handling
  and recovery mechanisms.
- `tests/examples.rs`: Verifies the execution of example scripts.

This reorganization improves test discoverability and maintainability.
The `lib.rs` file has also been updated to reflect these changes by
removing the reference to the old `integration_test` module.
The `type_checker.rs` file has had some tests removed, as they are now
covered by the more specific tests in `tests/error_recovery.rs`.

tests/pipeline.rs

@@ -0,0 +1,34 @@
+use myc::ast::environment::Environment;
+use myc::ast::types::Value;
+
+#[test]
+fn test_pipeline_optional_type() {
+    let env = Environment::new();
+    // The lambda uses an `if` without an `else` returning a float constant.
+    // The TypeChecker should deduce `Optional(Float)` for the lambda body,
+    // and correctly unwrap it to `Series(Float)` for the pipeline output.
+    let source = "(do
+        (def src (create-random-ohlc 42 10))
+        (def filtered
+            (pipe [src]
+                (fn [tick]
+                    (if true
+                        42.0
+                    )
+                )
+            )
+        )
+        filtered
+    )";
+    let res = env.run_script(source);
+    if let Err(e) = &res {
+        panic!("Script failed to compile/run: {:?}", e);
+    }
+
+    let val = res.unwrap();
+    if let Value::Object(obj) = val {
+        assert_eq!(obj.type_name(), "StreamNode");
+    } else {
+        panic!("Expected an Object(StreamNode)");
+    }
+}