Add File::read_contents and File::write_contents convenience functions.

Before:

```rust
use std::fs::File;
use std::io::Read;

let mut bytes = Vec::new();
File::open(filename)?.read_to_end(&mut bytes)?;
do_something_with(bytes)
```

After:

```rust
use std::fs::File;

do_something_with(File::read_contents(filename)?)
```

src/libstd/lib.rs

@@ -269,6 +269,7 @@
 #![feature(core_intrinsics)]
 #![feature(dropck_eyepatch)]
 #![feature(exact_size_is_empty)]
+#![feature(file_read_write_contents)]
 #![feature(fixed_size_array)]
 #![feature(float_from_str_radix)]
 #![feature(fn_traits)]