More readable next statement in IR syntax

Currently the `next` list in XLS IR is hard to read, because it is a list of values with no reference to the params being updated. If you want to, say, remove a param from a proc that's in the middle of the list, it is very tedious. I think syntax like this would be easier to read and modify with only modest overhead:

```
top proc main(tkn: token, foo: bits[32], bar: bits[32], baz: bits[32], init={1, 2, 3}) {
send.1: token = send(tkn, foo, channel_id=0)
next send.1 {
foo = bar
bar = baz
baz = foo
}
}
```

rather than

```
top proc main(tkn: token, foo: bits[32], bar: bits[32], baz: bits[32], init={1, 2, 3}) {
send.1: token = send(tkn, foo, channel_id=0)
next (send.1, bar, baz, foo)
}
```