Naming of binaries currently very generic

The tutorial mentions binary names like `repl` or `codegen_main`, which are pretty generic names.

For a future `bazel run :install` that puts it in `/usr/bin/` these should probably get names that reflect what they do. Similar to `xlscc`, using that prefix might already be good (e.g. `xlsrepl`, `xlscodegen`), in particular as it provides for ``-completable discovery of the available names.