TryInto

Great crate! I would love to see some support for unary enums in situation like this

```
custom_derive! {
    #[derive(...)]
    enum MyEnum {
        u8(u8),
        u16(u16),
    }
}
fn foo(x: MyEnum) -> Result<u8, SomeError> {
    let x:u8 = try!(x.try_into());
    x+1
}
```

So this is a bit like `conv`'s `TryFrom!` macro but for unary enums.
