Figure out how to make C structures and assign them to objects.

[mailund]

I have no idea if this is possible in C extensions, but considering that Rcpp can work on classes, it must be at least partially possible.

I probably cannot define actual struct declarations on the fly, but it shouldn't be a problem to do it manually through arrays with the right number of pointers. After all, I do not expect to put general values in pmatch-defined types.