Pass in fee payer as public key

[mitschabaude]

• closes Pass in fee payer as a public key #635 (see that issue for new Mina.transaction API)
• Remove private key argument to AccountUpdate.fundNewAccount(). Also, add an argument to fund multiple accounts and deprecate the initialBalance argument (can achieved in a cleaner way by calling .send() on the returned account update)
• update examples throughout to reflect these new APIs; also removes other deprecated uses of private keys in transactions (createSigned, this.sign()) along the way
• closes API to expose public key of caller in SnarkyJS methods #47
• new API to get the sender's public key from zkApp methods: this.sender. Also available globally as Mina.sender()
• try out this.sender in the dex examples

TODO:

• support public key as fee payer argument
• deprecate private key as fee payer argument
• update examples
• changelog

[Trivo25]

lgtm!

[ymekuria]

LGTM!

[ymekuria]

LGTM! I created an issue to update the example contracts in the CLI once this PR lands and a new version if SnarkyJS is published to npm.