Split blocks and closures (#7075)

* Split closures and blocks

* Tests mostly working

* finish last fixes, passes all tests

* fmt

src/tests/test_engine.rs

@@ -107,7 +107,7 @@ fn missing_flags_are_nothing4() -> TestResult {
 #[test]
 fn proper_variable_captures() -> TestResult {
     run_test(
-        r#"def foo [x] { let y = 100; { $y + $x } }; do (foo 23)"#,
+        r#"def foo [x] { let y = 100; { || $y + $x } }; do (foo 23)"#,
         "123",
     )
 }