new experiments
serialization of R objects using RDS for being able to reconstruct the call for callbacks
fixed callback to R with a parameter rather than a local variable. No need to load
have the create_foo_expression routine return the expression. This  will be the same as the deserialize approach and we can use them interchangably
assign the value from create_foo_expression to the global variable. Redundant, but fine
llvmAddSymbol() for R_loadRObjectFromString when compiling the deserialize routine
material around finding global variables
tidy up