Introduce World, which is a poorly named configuration.
Make code fetcher dependencies explicit.
Get rid of FakeFs and Pathname.

FakeFS is not RBX friendly and caused troubles several times. Pathname
consistency is hard to enforce and most input expects string anyway.
Remove loosely related compoenents and overcomplicated examples.

This is preparation for initial release. I think now that more specialized
components should live outside Core, though not sure about code fetchers
and vcs adapters.
Drop usecase suffix from what serves as main usecase now.
Move spec to correct namespace.