Fix coerceToTrueOrFalse to work correctly in the case that asLogicalNoNA()
returns invalid values.
Fix handling of 'return' when evaluating an EXPRSXP.
Automatically upcast parameters to runtime function calls as needed.
'return()' should return NULL, not void.
Add cxxr_runtime_loopFunctionIsNext to the runtime interface.
Fix the argument handling in the inline code generated for looping functions.