Add definition template for carpool contract

[pmiklos]

The carpool contract is relatively big and it costs about 1500 bytes when spending the funds from the contract due to the inclusion of the contract definition. The introduction of definition template would greatly reduce the costs since it the actual contract would only refer to the template hash and the actual parameters.

The template has 4 parameters:

• carpool oracle address
• carpool payout processor address
• driver's payout address
• passenger's refund address

The carpool oracle address and payout processor address are relatively stable and not expected to change, so they could be hard-coded into the template to further save cost.

[pmiklos]

the GUI wallet cannot handle smart contracts with definition template unfortunately as of today, so when a user receives such a contract, the payment request is shown, but clicking on it does nothing.