Better handle existential deposits in transfers

[steinerkelvin]

Currently, we are subtracting the "existential deposit" from the transfer amount before doing transfers.

This should be handled in another way.

   def transfer(
            self,
            key: Keypair,
            amount: int,
            dest: Ss58Address,
    ) -> ExtrinsicReceipt:
    # ...
    amount = amount - self.get_existential_deposit()