Have some form of destructing if

Having to constantly  write code like this:

```
alt optional_value {
  some (value)  { do something; }
  _ {/*do nothing*/}
}
```

Is unnecessary verbose in my opinion, and gets annoying to have to constantly write.  I would like to be able to say something like:

```
if (let some(value) = optional_value) {
  do something;
}
```

Although it doesn't have to be that exact syntax.
