Undeprecate and update bytes! macro.

1. Undeprecate bytes!
2. Make it possible to use bytes! on the RHS of a const.

See https://github.com/rust-lang/rfcs/pull/69#issuecomment-46071307 and the following for a discussion.

This will now expand to
```rust
{
    const BYTES: [u8, ..N] = [...];
    &BYTES
};
```