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

Compatible with update at circomlib
(cf853c1cc9)

v0.0.7

On a Intel(R) Core(TM) i7-8705G CPU @ 3.10GHz, with 32 GB of RAM:

- old:
hash  time:   [85.766 us 85.797 us 85.830 us]

- new:
hash  time:   [29.319 us 29.327 us 29.336 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]