Adapted FastR to Source.getCode() deprecation and CharSequence returned from Source.getCharacters().

The makefiles for this project don't support running with multiple jobs
(-j ...). This tells mx not to try to use `-j`.
Also the `native` attribute is a boolean.

Refactor ffi impls into com.oracle.truffle.r.ffi.impl project; add ffi AP; TruffleLLVM all RFFI interfaces implemented; no JNI

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.

Implemented TruffleRLanguage.toString(RContext context, Object value) and isVisible() to work properly in GraalVM shell and Interop.