first step: resolve compilation errors
fixed a latent bug not related to this refactoring
fixed missing conversions from Sort2 to string in Collection.contains/get methods
fixed method calls of the form: Sort2.equals(String)
fixed copyright
fixed compilation errors after rebasing
fixed more bugs
forgot to implement Serializable for the new sort class
fixed more bugs using search pattern context.*.contains*(*getSort())''
fixed two more bugs
fixed MetaK#isBuiltinSort/isDataSort
removed KSort; moved MetaK.isKSort/isComputationSort to Sort2
moved MetaK.isDataSort/isBuiltinSort to Sort2
changed Context#listConses to Map<Sort2, Production>
do the similar things for a few more public fields in Context
made Poset parametric
changed the type of Context#subsorts to be Poset<Sort2>; modified API's accordingly
changed the internal data of Sort from String to Sort2
cleaned up the Sort class
renamed Sort to NonTerminal, and Sort2 to Sort