Do root finding concurrently with concurrent.futures.ProcessPoolExecutor
Do root finding concurrently with concurrent.futures.ProcessPoolExecutor

This gives about a 1.8x speedup for one iteration of n=30 with prec=1000 and
divisions=200 (which isn't enough btw).

One issue here (aside from the added code complexity) is that the debug logs
from the root solving are printed out of order.

ergs/transmutagen,ergs/transmutagen