Java backend: minor: more documentation
Java backend: renamed CollectionVariable to ConcreteCollectionVariable to be precise
Java backend: brought back the cell-sorting pass from the history (not yet incorporated in the Java backend)
Fixed a bug that resulted in crash during syntax concretization
implemented a missing shallowCopy() method to avoid crash when doing syntax concretization
Java backend: lots of minor preparation work before incorporating cell sorting
Refactoring: slightly changed the signatures of constructors/factory methods in subclasses of DataStructureBuiltin to make the order of their arguments consistent