Split CppFunction into CppFunctionBase (for use with InternalFunction) and CppFunction (for use in a Module)
Allow InternalFunction to be constructed using a std::function
Since there's no good compile-time check for std::function support, this is guarded by a RCPP_USE_STD_FUNCTION macro, which has to be set before including any Rcpp header.
Merge pull request #162 from cauthmann/std_function

Support std::function in InternalFunction