My attempt at unslushing S11

Highlights:
- Introduce the "unit" statement, with :auth, :ver and :created adverbs
- Emphasize the use of compilation unit, or "compunit"
- Remove "auth" and "ver" from package like statements
- Remove section "Importing from a pseudo-package"
- Try to disambiguate "module" and "package" into "distribution",
  "compunit" and "namespace"
- various other tweaks