Fixed discovered issues.

1. Plugin systems now supports all three targets:
   - Native code
   - Bytecode
   - Toploop (I was surprised to discover, that it has different loading
      facilities)

2. Plugins are automatically loaded, when `bap.top` library is loaded.
   This means that, Plugins.load shouldn't be called.

3. CI will now catch lost files

4. Will now test that baptop loads correctly and can execute all
   supplied examples. Also, we now have an examples subfolder.