add RSymbol (symbol) type

...truffle.r.nodes/src/com/oracle/truffle/r/nodes/RNode.java

...le.r.nodes/src/com/oracle/truffle/r/nodes/RProxyNode.java

...ruffle.r.nodes/src/com/oracle/truffle/r/nodes/RTypes.java

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

...racle/truffle/r/nodes/builtin/base/PrettyPrinterNode.java

....nodes/src/com/oracle/truffle/r/nodes/builtin/base/R/as.R

...src/com/oracle/truffle/r/nodes/builtin/base/R/namespace.R

...s/src/com/oracle/truffle/r/nodes/builtin/base/Typeof.java

...ser/src/com/oracle/truffle/r/parser/ast/FunctionCall.java

....r.runtime/src/com/oracle/truffle/r/runtime/RRuntime.java

...e/src/com/oracle/truffle/r/runtime/data/RDataFactory.java

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