Problem: now that we have many packages in the repo, the root
directory became quite big, it is a bit hard to see what is there.
Also the name of `prelude` folder is a bit inconsistent: for other
packages their folder has the same name as the package, but for
`morley-prelude` it does not hold.
Solution:
1. Move all packages except for `morley` into `code/` directory.
The idea is to have all code there and `morley` will be moved in
the next commit as it is more involved.
2. Rename `prelude/` folder to `morley-prelude/`.