`_DIGIT_NAILS` is defined as 4, meaning that we use 60 out of every 64 bits.
We can use as few as 1 nail, using 63 bits out of every 64, and all tests will still pass.
However, it needs more testing to see if that's a worthwhile change to make.
For the tests to work properly when changing the nails, Python needs to know about it as well.
In addition, compile the big math code with `-o:speed` going forward.