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.
Add C++ function to polynomial-expand a matrix row
Implement the beef of predict/predVals in C++
Rewrap comments and stop needlessly copying terms
Rename prototype predVals function
Add R wrapper around C++-based predict
Correct CI computation when avg = TRUE

Previously was adding XB/n_boot to each element of fit_boot within each
iteration.  But fit_boot has n_boot elements, each of which is visited
n_obs times, and the final CI is taken across bootstrap
iterations (since we're computing the grand mean prediction for each
one).  So we should divide by n_obs, not n_boot.
Add R wrapper around C++-based predVals
Add todo