Fix: Package 'jsonlite' expected CHAR(x) to return the same pointer on subsequent calls if x is the same. Now, strings are cached.

Added class NativeConnections represented such connections and performing native calls for reading and writing.
This changes make package "curl" working.

Also added a few more functions to access the properties of a connection.
The implementation was tested with R package 'jsonlite'.

* NULL the entry in nativeArrayTable instead of its copy
* updateNativeArrays cannot retain the buffers, because the underlying vector may be modified by the up-call

"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

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

RFFI: refactor upcalls into separate interfaces; fix R_Home/R_Interactive rewrites; make tempdir context sensitive, clean up CallRFFI