allow users to disable inclusion of modules code

[kevinushey]

The modules code in Rcpp (especially the 'code bloat') is responsible for a large chunk of the time spent during compilation, as the compiler has to do a lot of work to parse all of the generated functions.

What would we think about adding a define RCPP_NO_MODULES (similar to RCPP_NO_SUGAR) that would allow users to disable inclusion of the modules code, for cases where they know they don't need it?

[kevinushey]

It could also be worth looking into using variadic functions as a replacement when C++11 features are available there as well.