Support for 32-bit word width

[yoaveshel]

Are there any plans to support 32-bit words? I rewrote the division algorithm for a 32-bit machine and got an 18% speedup (testing was done on a risc-v simulator), so it seems there are some performance to be gained here.

[DaniPopes]

Note that this would be a breaking change, but it makes sense to have machine-word-sized limbs instead of u64.