Switch to Rcpp for the C++ API

[LTLA]

Wondering whether I should switch to Rcpp, in light of the recent discussion about changes to memory allocation and PROTECT philosophies in R's C API. There are two options for doing so:

• All-out refactoring to subclasses of the Rcpp::Matrix. This would be painful because we'd have to be sure that we properly implement all of the Matrix methods, conversions, etc., to honour what users might expect from a derived class. Most of these we probably don't need for basic data storage and access. It also seems that the Matrix class declaration doesn't have protected members/methods, only private ones; I don't think that we can actually get at internal variables like nrows.
• A "light-touch" approach where we just refactor the code so that the internal data objects use Rcpp, and the front-end is still under our control. This is relatively easy to do, notwithstanding my ignorance of Rcpp, and should ensure that the code is robust to changes in the need for PROTECT.

As the tone suggests, I favour the latter. But if an Rcpp expert is willing to help, we could do the former.

[LTLA]

Done, albeit in an ugly and highly non-idiomatic way. Basically just set up Rcpp::RObject data members for each class so that the pointers of the class are always pointing to PROTECTed data across the lifetime of the class. The API still returns pointers of values, we're not using the Rcpp row/column objects.