Closes#10. This combines Lagrange interpolation with FFTs as mentioned there.
I was previously thinking that all our polynomial encodings might as well just use power-of-two length vectors, so they'll be "FFT-ready", with no need to trim/pad. This sort of breaks that assumption though, as e.g. I think we'll want to compute interpolants with three coefficients in the batch opening argument.
I think we can still skip trimming/padding in most cases, since it the majority of our polynomials will have power-of-two-minus-1 degrees with high probability. But we'll now have one or two uses where that's not the case.
