RExpression extends RVector (instead of wrapping an RList)

...fle.r.engine/src/com/oracle/truffle/r/engine/REngine.java

...n/src/com/oracle/truffle/r/nodes/builtin/base/AsCall.java

...c/com/oracle/truffle/r/nodes/builtin/base/AsFunction.java

...src/com/oracle/truffle/r/nodes/builtin/base/AsVector.java

.../src/com/oracle/truffle/r/nodes/builtin/base/Combine.java

...c/com/oracle/truffle/r/nodes/builtin/base/Expression.java

...rc/com/oracle/truffle/r/nodes/builtin/base/Identical.java

.../oracle/truffle/r/nodes/builtin/base/IsTypeFunctions.java

.../com/oracle/truffle/r/nodes/builtin/base/ListBuiltin.java

...n/src/com/oracle/truffle/r/nodes/builtin/base/Mapply.java

...in/src/com/oracle/truffle/r/nodes/builtin/base/Parse.java

...uffle/r/nodes/builtin/base/printer/ExpressionPrinter.java

...ruffle/r/nodes/access/vector/CachedExtractVectorNode.java

...ruffle/r/nodes/access/vector/CachedReplaceVectorNode.java

...truffle/r/nodes/access/vector/WriteIndexedVectorNode.java

...src/com/oracle/truffle/r/nodes/builtin/RList2EnvNode.java

.../com/oracle/truffle/r/nodes/unary/CastExpressionNode.java

...es/src/com/oracle/truffle/r/nodes/unary/CastListNode.java

...com/oracle/truffle/r/nodes/unary/CastToContainerNode.java

...rc/com/oracle/truffle/r/nodes/unary/CastToVectorNode.java