Remove need for sprout-groth16.key download

[leto]

This 692MB file is not needed by Hush at all, but because librustzcash library is initialized by passing in Sapling keys (which are less than 50MB in total) AND the groth16 key, there is no way to initialize librustzcash without it.

This makes every Hush full node download almost 700MB for no reason, wasting bandwidth. This also increases the bandwidth bill of whoever pays to server the Zcash param files, for no good reason.

librustzcash should be able to init via ONLY sapling params files

[leto]

Requested this feature upstream here: zcash/librustzcash#85

[leto]

Some things to try:

• Is it possible to give librustzcash an empty file with correct hash for groth16 and perhaps ignore some errors/warnings?
• What would it take to make a new init function which does not take groth16 as an argument?

[leto]

Done! Will be merged from duke branch soon