compile stub files

[marionebl]

Compile stub files introduced via #285 against tests to ensure they stay in sync.
As per #306 (comment).
Depends on #356

• Add new dune alias to all exercises

(alias
  (name    buildtest)
  (deps    (:x test.exe)))

• Downgrade unused variables etc. to warnings instead of errors

(alias
  (name    buildtest)
  (deps    (:x test.exe)))

(env
  (dev
    (flags (:standard -warn-error -A))))

• Fix beer_song stub file by renaming lyrics to recite