allow serialization to insert constants to deparsed code

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

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

...e.r.nodes/src/com/oracle/truffle/r/nodes/RASTBuilder.java

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

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

...time/src/com/oracle/truffle/r/runtime/context/Engine.java

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

...acle/truffle/r/test/builtins/TestBuiltin_unserialize.java