Remove to_cstring() naming convention ambiguity

Add a trait for str.strdup() to replace to_cstring() which avoid the
signature ambiguity with .to_string().

Also instruduce CString::yolo() as a shortcut to
CString::new().unwrap() and use it whenever the variable does can be
deallocated by going out of scope.  This is less error prone.

Use some Path.to_c_string() functions where possible.

examples/repl/main.rs

@@ -481,7 +481,7 @@ unsafe fn handle_cmd(line: &str, ctx: Arc<RwLock<Context>>) -> Result<ExitResult
     let arg1_c = if arg1.is_empty() {
         std::ptr::null()
     } else {
-        to_cstring(arg1)
+        arg1.strdup()
     };
 
     match arg0 {