header-only C++ library

- [ ] Switch from autogeneration of header linking (slowed calls) to pure in-line embedding. 

Notes:
Remove `// [[Rcpp::interfaces(r, cpp)]]`

Build definitions in `inst/include`

`simcdm.h`
```
namespace simcdm{
   #include "sim_rrum.h"
}
```


Benefits:

Ship as a detached _C++_ library from _R_. 