`struct Field`

Making this the tracking issue for `struct Field`, an atomic storage field that knows its own address and memoizes its value.

Pseudo-example:

```rs
#[contract]
struct Contract {
admin: Field
}

impl Contract {
fn new (storage: Storage) -> Self {
Self { admin: Field::new(storage, KEY_ADMIN_ADDR) }
}
fn assert_admin (&self, env: Env) -> StdResult<()> {
if self.admin.get() != env.message.sender {
return Err(StdError::unauthorized())
}
Ok(())
}
}
```

I'd like to avoid having a `.get()` method, so `Field` should implement `Deref`/`DerefMut`