Improve performance by checking many files at once

Here are timings for `tests/underscore-any.ts`:

```
creteHost 22 ms
creteProgram 710 ms
getSource / createScanner 0 ms
getTypeChecker 471 ms
getPreEmitDiagnostics 932 ms
checkFile 4 ms
```

The three slow calls could all be sped up by loading many ts files in one batch.