This change follows the layout of our other projects, like dSS and dSA.

Third party code (with optional patches on top) is placed in a separate
directory; our sources are organized in subdirs named by functionality.

All tests reside in toplevel/tests.

This commit will not compile on it's own, there is no point to update
the makefiles now, because the next commit will rework the whole build
to avoid recursive make.