Adds a flag to CastBaseNode that indicates that it should behave in a way
compatible with coercions exposed by the R native interface.

"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

* commit '6427f5a1':
  Fixed casts in foreign Dqrdc2.

Merge pull request #693 in G/fastr from ~LUKAS.STADLER_ORACLE.COM/fastr:bugfix/check_arg_not_null to master

* commit 'ef0b63e3':
  check that arguments are not null

rffi; replace UpCallsIndex with RFFIUpCallMethod enum; fix bug in R_NewHashedEnv signature; simplify RForeignAccessFactoryImpl