Rewrite solcjs to use functions/callbacks

And in that case stdio reading can be done properly using:
```
process.stdin.on('data', function(chunk) {
})
process.stdin.on('end', function() {
})
```

Start at the solcjs command-line entry point that reads process.stdin and trace how input and completion are currently handled. Compare that flow with the data and end callbacks shown in the issue, then identify existing tests for standard-input compilation. Done means stdin is consumed reliably through callbacks and the CLI still produces the expected compiler output.

Written by the indexing model from the issue text.