Byte-compile modules

Does it make sense to byte-compile modules, either upon loading or (ideally) cached?

Of course, this would [not be as fast](http://dirk.eddelbuettel.com/blog/2011/04/12/) as using `Rcpp`, but is a speed increase that comes at (almost) no cost to the end user.

Start by investigating how R modules are loaded and whether byte-compilation can occur during loading or be cached. Compare the possible speed benefit with the Rcpp comparison mentioned in the issue, and define what caching behavior and performance improvement would count as done.

Written by the indexing model from the issue text.