Assembling doesn't work.

Eg, 

```rust
use std::io::*;
use x86asm::*;

let buffer = Cursor::new(Vec::new());
let mut writer = InstructionWriter::new(buffer, Mode::Long);
writer.write2(Mnemonic::MOV, Operand::Direct(Reg::RBX), Operand::Direct(Reg::RAX))?;
```

causes an `unimplemented` panic.