DSLX: Missing error check in proc member assignment

I have a test proc :
```
#[test_proc()]
proc Proc {
terminator: chan out;
bar: chan out;

config(terminator: chan out) {
let (foo, baa) = chan;

(terminator, bar)
}

next(tok: token) { () }
}
```

Note that the channel is declared as "baa", but the member assignment is "bar". Instead of a ParseError, we get a
`Could not find slot or binding for name: bar @ foo.x:5:3-5:6` error. I suspect that the...parser? doesn't catch this error b/c "bar" is considered in scope of `config()`, but it shouldn't be.