Added tests on labels
 #KT-1703 Fixed
 #KT-361 Fixed
 #KT-3920 Fixed
 #KT-3988 Fixed
 #KT-4247 Fixed
 #KT-4586 Fixed
 #KT-4603 Fixed
 #KT-591 Fixed
Don't parse @, @@ as independent tokens
Regenerated lexer
Removed AT, ATAT tokens
Checks 'in LABELS token set' replaced with LABEL_IDENTIFIER equality
Rename
JetLabelQualifiedExpression -> JetExpressionWithLabel
JetLabelQualifiedInstanceExpression -> JetInstanceExpressionWithLabel
Minor: added annotation, used util method
Added JetLabeledExpression; added support to parser
Replaced PrefixExpression with LabeledExpression
where labeled expression is used

Added 'visitLabeledExpression'
Generate parse error on label without name '@'