poly: add pass to convert poly.mul to use ntt ops

[inbelic]

• We can reduce complexity of polynomial multiplication by using the number-theoretic transform

• Here we add a pass that will convert all poly.mul operations into a poly.ntt of both operands -> modulus multiplication of the coefficients -> poly.intt back into the polynomial type

Resolves #541

[j2kun]

Also please squash the commits when you're ready. Thanks!

[inbelic]

Great thanks. I will be AFK until the 15th, but can fix it up when I have my laptop again. Otherwise, feel free to apply your suggestions and merge. Sorry for the delay