Optional initial expression for collection attributes

[Jesper Öqvist]

Collection attribute declarations could have the initial expression as an optional part of the syntax. A default initial expression would be just calling the constructor of the collection type with no argument.

For example:

coll LinkedList<Integer> N1.c() with add;

would be equivalent to

coll LinkedList<Integer> N1.c() [ new LinkedList<Integer>() ] with add;

[Jesper Öqvist]

Make coll initial expression optional

fixes #232 (bitbucket)

→ <<cset 8ebdc36f7faa>>