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

* 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

rffi: add interface for RFFI upcalls; refactor native layer to use instance not static class; add factory for alternate upcall implementations

refactor TestRFFIPackage, add more tests; (pessimally) fix “complete” status of vectors updated in RFFI

Interning of ReadVariableNode ids and RSymbol names to make sure that string comparisons work correctly.