Make stream creation atomic

[TomAFrench]

This PR consolidates the two transactions to create a stream (sending ZkAsset to contract and initialising stream) into one.

Currently blocked as the AZTEC SDK doesn't support signing an approval for the contract to submit the JoinSplit proof. The approval could be generated outside the sdk could be a worrying change to the standard flow for users.

closes #25

Tasks before merging:

• [x] test failure conditions in StreamUtilities.sol
• [ ] test success outcomes of StreamUtilities.sol functions
• [ ] update frontend to call new sdk method to get signed proof
• [ ] deploy new contract to rinkeby to close #41

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