implement KonohaByteCodeGen API

revision previous pull request
-Use LibGreenTea.QuoteString(ConstValue),
-Emit NUL bytecode in VisitNullNode,
-methodnize routine of get offset and implement VisitGetterNode
add field for Label
change VisitReturnNode for void type func
implement VisitIfNode, Visit(While|DoWhile|For)Node,
Visit(And|Or)Node, and Visit(Continue|Break)Node