Use Vec<u8> for LOCAL_STD{OUT,ERR} instead of dyn Write.

It was only ever used with Vec<u8> anyway. This simplifies some things.

- It no longer needs to be flushed, because that's a no-op anyway for
  a Vec<u8>.

- Writing to a Vec<u8> never fails.

- No #[cfg(test)] code is needed anymore to use `realstd` instead of
  `std`, because Vec comes from alloc, not std (like Write).

library/std/src/lib.rs

@@ -297,6 +297,7 @@
 #![feature(raw)]
 #![feature(raw_ref_macros)]
 #![feature(ready_macro)]
+#![feature(refcell_take)]
 #![feature(rustc_attrs)]
 #![feature(rustc_private)]
 #![feature(shrink_to)]