[IFT] major rework to the IFT API around patch intersection and application.

[garretrieger]

Adds a new patch_group module which generates a candidate set of URIs to be applied next based on the list of intersecting patches. The candidate URI group is structured to enforce compatibility constraints between patches. PatchGroup's can then be used to trigger the next iteration of patch application.

Example usage:

loop {
  let g = PatchGroup::select_next_patches(font, &subset_definition).unwrap();
  if !g.has_uris() {
    break;
  }

  // g.uris() lists patches to be downloaded, 
  // patch_data is populated with downloaded data.
  let patch_data = ...;

  font = g.apply_next_patches(&mut patch_data).unwrap();
}

[cmyr]

I have no familiarity with the IFT spec, but reading through this nothing strikes me as egregious. I've left some little rust style nits inline.