`@spawn` has confusing move semantics

Note that
```
◖◗ Dagger.@spawn single=2 println(myid())
EagerThunk (running)

◖◗ From worker 2: 1
◖◗

◖◗ Dagger.spawn(() -> println(myid()), single=2)
EagerThunk (running)

◖◗ From worker 2: 2
```
I think what's happening here is that in the former case it is evaluating `myid()` on process 1 and the function executed on worker 2 is just `println(1)`.

This seems like a problem, it's hard to guess that these examples would be different. I suggest that `@spawn` should match `spawn` so that its arguments can be read as if it were the function body passed to `spawn`. Otherwise I think it will need a lot of documentation.

A stronger argument might be that `Distributed.@spawnat` already works as I'm suggesting:
```
◖◗ @spawnat 2 println(myid());

◖◗ From worker 2: 2
```

(My suggestion is clearly badly breaking at least in principle, so this is tricky.)