std: Rename the ToStr trait to ToString, and to_str to to_string.

[breaking-change]

src/librustdoc/lib.rs

@@ -367,7 +367,7 @@ fn json_input(input: &str) -> Result<Output, String> {
         }
     };
     match json::from_reader(&mut input) {
-        Err(s) => Err(s.to_str()),
+        Err(s) => Err(s.to_string()),
         Ok(json::Object(obj)) => {
             let mut obj = obj;
             // Make sure the schema is what we expect