Type map c/c++ : const reference as argument but not as output.

[jmp75]

Consider an API function with

	SWIFT_API values_vector* ConvertValues(values_vector* values);

The argument and returned values are pointing to different objects. The RCpp wrapper function should be signed:

NumericVector ConvertValues_RcppPostfix(const NumericVector& values)

It (usually) makes sense to have const ref in but not out. Right now, the type mapping is a bijection so this is hard to support this. This may require a substantial refactoring of the tool.