Implement methods for sp types

[noamross]

This should be pretty simple, just convert sp to sf. Need to set up S3 dispatch for this.

[mpadge]

@noamross seems like a good place to cross-fertilise your grand work on fasterize with osmdata - submitted to ropensci. You're going from sf to other formats; osmdata does conversions in the other directions, absent direct Depends or Imports of the beast that is sf.

Full sp code is also included here - feel free to crib all you like, but note that Rcpp dis-mantling of S4 objects is considerably slower than simple S3 ones, so your results aren't likely to be as impressive as for sf. osmdata has some timing comparisons here, which don't even include sp because it's orders of magnitude slower.