Move everything to one source folder (closes #29)

(Except source-gen)
Fix source-gen bug: doc would be missing line breaks
source-gen: Generate backend toCeylon + compile functions
Add GroupedType backend tests

No idea why they were missing…
Remove unneeded imports (same package, no rename)

They were probably originally added by the IDE when I copied test code
from one file to the other (as boilerplate), and then I copied the
entire file as test code boilerplate instead, and the import got carried
over. Gah.
source-gen: Generate backend conversion + compilation test