[xbase][typesystem] Improved type inference for collection literals

Also fixed assignability issue with nested type arguments.

see https://bugs.eclipse.org/bugs/show_bug.cgi?id=435473
see https://bugs.eclipse.org/bugs/show_bug.cgi?id=455317

Change-Id: I0e7d7cd1ba4c98b583594c0c9966e91eb6b8a04b