Fixing issue #549. Sometimes a non typed variable in a KApp wouldn't be wrapped with a KLabel object.
   - added a check which ads it after disambiguation.
Checking only for the sort information.
Merge pull request #593 from kframework/fix-i549

Fix i549, latest fix worked.
Merge pull request #551 from yilongli/use-cons-list-in-ksequence

Use cons list in ksequence
cleaned up the constructor of KItem; cached LIST_TERMINATOR