Consolidate attribute-related functionality in RAbstractContainer interface.

...ime/src/com/oracle/truffle/r/runtime/data/RDataFrame.java

...me/src/com/oracle/truffle/r/runtime/data/RExpression.java

...untime/src/com/oracle/truffle/r/runtime/data/RFactor.java

...time/src/com/oracle/truffle/r/runtime/data/RLanguage.java

...time/src/com/oracle/truffle/r/runtime/data/RPairList.java

...time/src/com/oracle/truffle/r/runtime/data/RSequence.java

...cle/truffle/r/runtime/data/closures/RToVectorClosure.java

...acle/truffle/r/runtime/data/model/RAbstractContainer.java

.../oracle/truffle/r/runtime/data/model/RAbstractVector.java