Use ,commands. Enhance log command.

- Instead of `(run!)`, `(doc!)` etc. use `,run`, `,doc` etc. Like xrepl.

- Enhance the log command to work like my outstanding pull-request for
  xrepl: https://github.com/plt/racket/pull/188
If can't create module evaluator, do top-level rather than exit.
Forget compilation errors.

That way, previous errors will be ignored by C-x ` next-error.