Avoid field inverse for r == x comparison

As suggested by @gmaxwell. This seems to give a 3% speedup for verification.

Builds on top of #119.
Do you mean `&& xr == x(R) + h * order)` (and then to change the rest of the lines)? Otherwise I don't see this -- it seems like you're checking that `x(R)` won't overflow but then adding to `xr` rather than `x(R)`.