Move prototype code to subdirectory
Implement quantile in C++
Remove C++ quantile implementation

The hand-rolled implementation takes longer than the native R version,
per microbenchmark(), so I may as well just call the R version from
within the C++ code
Add C++ function to compute polynomial expansions

This function works row-by-row, so that we never have to store the
entire N x polynomial-terms(k) matrix in memory at once.  It is not
meant to be called directly by the user and will probably end up in a
header file in the final rewritten version of the package.