This adds mustBeValidVarArgs method to the cast pipeline

(cherry picked from commit f29e18bc)

* New interface RootBodyNode implemented by FunctionBodyNode
* FunctionBodyNode saves args and then executes the actual body node
* New tag FunctionBodyBlockTag that tags BlockStatements that are function bodies
* FastR debugger that used to listen to RootTags (originally BlockNodes that had FunctionDefinitionNode as parent) is now listening to FunctionBodyBlockTags

- fixes

fake registration of a RFunction as if it was a native function callable by .C/.Call/.External/.Fortran