Libfunc for buying a coupon for a function. The cost of the coupon is the cost of running the
function (not including the call and ret instructions).
The coupon can be used to pay in advance for running the function, and run it later for
free (paying only for the call and ret instructions) using coupon_call.
Concrete type information for Coupon<function>.
Libfunc for getting a refund for an unused coupon. The refund is the cost of the function
and it is added back to the gas wallet.
Coupon type Coupon<function> (function::Coupon) which represents that the cost of a
function was paid, without calling the function yet.
Using the coupon the function can be called without paying the cost.