[GR-2798] Implemented builtins 'comment' and 'comment<-'.

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

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

...om/oracle/truffle/r/nodes/builtin/base/UpdateComment.java

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

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

...est/src/com/oracle/truffle/r/test/ExpectedTestOutput.test

...m/oracle/truffle/r/test/builtins/TestBuiltin_comment.java

...le/truffle/r/test/builtins/TestBuiltin_commentassign.java