following a86654785d

Update Poseidon to last changes of the reference implementation from
26ddaa91db

Compatible with update at circomlib
(cf853c1cc9)

On a Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz, with 16GB of RAM:
- Old (using num-bigint):
hash  time:   [5.9258 ms 5.9407 ms 5.9587 ms]
- New (using ff):
hash  time:   [120.12 us 121.08 us 122.30 us]

On a Intel(R) Core(TM) i7-8705G CPU @ 3.10GHz, with 32 GB of RAM:
- Old (using num-bigint):
hash  time:   [4.1192 ms 4.1318 ms 4.1461 ms]
- In the previous commit (using ff):
hash  time:   [91.394 us 91.430 us 91.476 us]
- In this commit (using ff):
hash  time:   [85.517 us 85.545 us 85.574 us]

Tested on a Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz, with 16GB of RAM.

- Old (using nnum-bigint):
hash           time:   [5.9258 ms 5.9407 ms 5.9587 ms]

- New (using ff):
hash           time:   [120.12 us 121.08 us 122.30 us]