auto merge of #18463 : japaric/rust/bytes2, r=alexcrichton

- The `BytesContainer::container_into_owned_bytes` method has been removed

- Methods that used to take `BytesContainer` implementors by value, now take them by reference. In particular, this breaks some uses of Path:

``` rust
Path::new("foo")  // Still works
path.join(another_path) -> path.join(&another_path)
```

[breaking-change]

---

Re: `container_into_owned_bytes`, I've removed it because

- Nothing in the whole repository uses it
- Takes `self` by value, which is incompatible with unsized types (`str`)

The alternative to removing this method is to split `BytesContainer` into `BytesContainer for Sized?` and `SizedBytesContainer: BytesContainer + Sized`, where the second trait only contains the `container_into_owned_bytes` method. I tried this alternative [in another branch](https://github.com/japaric/rust/commits/bytes) and it works, but it seemed better not to create a new trait for an unused method.

Re: Breakage of `Path` methods

We could use the idea that @alexcrichton proposed in #18457 (add blanket `impl BytesContainer for &T where T: BytesContainer` + keep taking `T: BytesContainer` by value in `Path` methods) to avoid breaking any code.

r? @aturon 
cc #16918

src/test/run-pass/process-spawn-with-unicode-params.rs

@@ -48,7 +48,7 @@ fn main() {
 
         let child_filestem = Path::new(child_name);
         let child_filename = child_filestem.with_extension(my_ext);
-        let child_path     = cwd.join(child_filename.clone());
+        let child_path     = cwd.join(child_filename);
 
         // make a separate directory for the child
         drop(fs::mkdir(&cwd, io::USER_RWX).is_ok());