Implement Sinsemilla hash

[jules]

[jules]

To note:

Sinsemilla with the default parameters k := 10 yields 1537 constraints for a 1030 bit input. Sinsemilla with k := 14 yields 1102 constraints for a 1036 bit input.

The math needs to be checked on whether we can modify the window size, but we can really drop a lot of constraints if it turns out to be the case, so an investigation is worth it.

[jules]

Testing constraint counts for both bit windows:

sinsemilla 14 bit window:
1024 (1036) yields 1102
768 (770) yields 817
512 (518) yields 547
256 (266) yields 277

sinsemilla 10 bit window:
1024 (1030) yields 1537
768 (770) yields 1147
512 (520) yields 772
256 (260) yields 382

I'll do some more research to figure out if this is safe to do or not.