feat: add single file string.rs

single_file_tests/string.rs

@@ -0,0 +1,11 @@
+fn main() {
+    let mut hello = String::from("hello");
+    hello.remove(3);
+    println!("removed  : {}", hello);
+    hello.pop();
+    println!("poped    : {}", hello);
+    hello.truncate(1);
+    println!("truncated: {}", hello);
+    hello.clear();
+    println!("cleared  : {}", hello);
+}