Renamed b/brainf*ck.bf to b/brainf_ck.bf to avoid crossplatform problems

The filename `brainf*ck.bf` causes problems on Windows. I replaced `*` with underscore `_`.

The problem was quite interesting: how to checkout file and rename it at the same time?

The only solution I found was:

* delete the file and commit,
* create new file and commit,
* join both commits with rebase.