Add the possibility to run the platform tests in Mozart 2

Following the very helpful comment in #67, here are the described changes as a PR.
It would be quite important to have some kind of backtrace for failing tests, right now the backtrace only contains the first line of the functor in `Init.oz`. @sjrd, @yjaradin any idea?
This PR does not try to make it integrated with the build system.

I also added a `simple_runner.oz` (which location is open to discussion, should it be in lib/ or where it is so more visible and intuitive?). It has the huge advantage of requiring only a single invocation to evaluate a changed test file (.oz) instead of 3 with the legacy runner. Also it can run compiled tests (.ozf) directly and consecutively, without creating any file in the current directory and needing to run them in another process.

The tests `base/{conversion,dictionary,exception,float,int,record,state,type}.ozf` pass, (`record` without the RecordC tests, and commenting one line of `type` for Unicode strings).

What do you think?