Channel declarations in config funtions should return a struct in DSLX

Right now in the `config` of a proc chan declarations produce a two-tuple of in and out channels which can be used when spawing other procs. Exmaple:

```
pub proc foo {
in_ch: chan in;
...
config (a: chan in) {
let (x_in, x_out) = chan;
spawn bar(x_in);
spawn qux(x_out);
(a, )
}
next (...) { }
}
```

A cleaner syntax might be for `chan` to return a struct containing the in and out channels. So you'd have:

```
pub proc foo {
in_ch: chan in;
...
config (a: chan in) {
let x = chan;
spawn bar(x.in);
spawn qux(x.out);
(a, )
}
next (...) { }
}
```