Add remove/insert to arrays and maps.

tests/maps.rs

@@ -33,6 +33,16 @@ fn test_map_indexing() -> Result<(), EvalAltResult> {
     assert!(engine.eval::<bool>("let y = #{a: 1, b: 2, c: 3}; 'b' in y")?);
     assert!(!engine.eval::<bool>(r#"let y = #{a: 1, b: 2, c: 3}; "z" in y"#)?);
 
+    assert_eq!(
+        engine.eval::<INT>(
+            r#"
+                let x = #{a: 1, b: 2, c: 3};
+                let c = x.remove("c");
+                x.len() + c
+           "#
+        )?,
+        5
+    );
     assert_eq!(
         engine.eval::<INT>(
             r"