"testthat" has been tested running tests of package "argparse".

- implemented RFFI functions PRSEEN, PRENV, R_PromiseExpr, and PRCODE
- cleanup of method "getIntrinsicValue"; replaced by RASTUtils.createLanguageElement
- added dummy cast to node IsATTY
- also fixed FileConnections.writeString; was required to run a package
- added tests for R exception handling; since we still have bugs, some tests are ignored
- also fixed a problem during source code parsing in REngine, which occurred if the code contained empty lines that have not been suppressed by any shell

Adapted to changes in GR-1767: frame parameter removed from interop functions and transitively from all callers where possible.
Also updated Truffle version in file "suite.py".
Removed class TruffleRFFIFrameHelper since there are no usages any longer.
Removed unnecessary call to CompilerDirectives.transferToInterpreter call in class ValuePrinterNode.

Sessions could not be save for empty workspaces since LoadSaveFunctions.SaveToConn expected a non-empty list of variables to store. However, GnuR allows to do so.
Also fixed problem when using function "q()" in test cases which caused the test suite to crash due to an unexpected but legitimate exception.