Using hash_tree_root for DepositData

[hwwhww]

What was wrong?

Fix #29
Goes along with spec PR #924

How was it fixed?

1. Per Using hash_tree_root for DepositData #29 (comment) suggestion, the API now is: def deposit(pubkey: bytes[48], withdrawal_credentials: bytes[32], signature: bytes[96])
2. Add minimal_ssz.py in this repo to verify tree hashing, but I suppose we will either use updated py-ssz or move deposit contract back to eth2.0-specs.

Gas used of the first 10 deposits

Before

deposit transaction consumes 144374 gas
deposit transaction consumes 116018 gas
deposit transaction consumes 114448 gas
deposit transaction consumes 117662 gas
deposit transaction consumes 114448 gas
deposit transaction consumes 116092 gas
deposit transaction consumes 114458 gas
deposit transaction consumes 119370 gas
deposit transaction consumes 114448 gas
deposit transaction consumes 116028 gas

After

deposit transaction consumes 187771 gas
deposit transaction consumes 144415 gas
deposit transaction consumes 142781 gas
deposit transaction consumes 146059 gas
deposit transaction consumes 142781 gas
deposit transaction consumes 144425 gas
deposit transaction consumes 142791 gas
deposit transaction consumes 147703 gas
deposit transaction consumes 142781 gas
deposit transaction consumes 144425 gas

Increased ~28,000 gas :'(

Cute Animal Picture

[NIC619]

LGTM!