Attributes should be accessed via message sends with slightly different behavior identifiers

[mjstahl]

+ (f Foo) bar ->
  'Get the attribute hello from object f and assign it to a',
  a := f @hello.

+ (b Bar) baz ->
   'Assign the attribute world on object b the value 5',
   b @world: 5.

[mjstahl]

Reference commit deb95c6 as the first commit on this issue (forgot to add it to the commit message)