implement Writer for Vec<u8>

The trait has an obvious, sensible implementation directly on vectors so
the MemWriter wrapper is unnecessary. This will halt the trend towards
providing all of the vector methods on MemWriter along with eliminating
the noise caused by conversions between the two types. It also provides
the useful default Writer methods on Vec<u8>.

After the type is removed and code has been migrated, it would make
sense to add a new implementation of MemWriter with seeking support. The
simple use cases can be covered with vectors alone, and ones with the
need for seeks can use a new MemWriter implementation.

src/librustdoc/lib.rs

@@ -28,7 +28,7 @@ extern crate "test" as testing;
 #[phase(plugin, link)] extern crate log;
 
 use std::io;
-use std::io::{File, MemWriter};
+use std::io::File;
 use std::collections::HashMap;
 use std::collections::hash_map::{Occupied, Vacant};
 use serialize::{json, Decodable, Encodable};
@@ -467,12 +467,12 @@ fn json_output(krate: clean::Crate, res: Vec<plugins::PluginJson> ,
     // FIXME #8335: yuck, Rust -> str -> JSON round trip! No way to .encode
     // straight to the Rust JSON representation.
     let crate_json_str = {
-        let mut w = MemWriter::new();
+        let mut w = Vec::new();
         {
             let mut encoder = json::Encoder::new(&mut w as &mut io::Writer);
             krate.encode(&mut encoder).unwrap();
         }
-        String::from_utf8(w.unwrap()).unwrap()
+        String::from_utf8(w).unwrap()
     };
     let crate_json = match json::from_str(crate_json_str.as_slice()) {
         Ok(j) => j,