Is this really different enough that it deserves to be deferred for a future RFC? Because if you're just looking to avoid a syntax bikeshed...
```rust
spawn(proc(x) { ... });  // old
spawn(once |x| { ... });  // new
```
...then IMO this looks like it would be an improvement. Still not quite as pretty as the old `do spawn`, but I was never comfortable with how non-closurelike the `proc` syntax looks. :P