Generate a sapling key independently of contract

Fix #147 (closed)

Creating a sapling key can be done independently of a contract. The flow is now:

• tezos-client sapling gen key test
• tezos-client remember contract contract_test [address]
• tezos-client sapling use key test for contract contract_test

Multiple keys can also be generated and be used for a smart contract

• tezos-client sapling gen key test2
• tezos-client sapling use key test2 for contract contract_test

Balances can also be fetched independently (was always the case):

• tezos-client sapling get balance for test --for-contract contract_test

A big change is the same key can be used for multiple contracts:

• tezos-client remember contract contract_test2 [address]
• tezos-client sapling use key test for contract contract_test2

TODO:

• change existing tests according to the new flow
• add tests
  • gen + use
  • deploy two shielded-tez contract, create transactions, check the balance are fetched correctly. This is done in the ShieldedTez class, the last test