PassAll gas option for program to program calls

[iFrostizz]

Closes #1184

Create a new Gas type that replaces the current alias by an enum that can either be PassAll or Units(NonZeroU64). The new PassAll works around the current way of spending gas, namely calling ConsumeFuel with the passed Fuel parameter and refunding any unused gas by calling AddFuel after the execution.

[samliok]

I think this PR's functionality is important but I don't think a Gas enum is the best way to do it. It seems much more intuitive to have gas be a field of context. This way we can keep the u64 type(which is nice for dev experience and contract readability) and also have access to the actual amount of gas via the context.

[iFrostizz]

Good point @samliok ! Thanks

[iFrostizz]

Actually I'm not sure if that's totally correct. We already have the remaining_fuel method in the implementation of Program but it's not gonna work because it's the full amount of gas is passed and there isn't really any way to predict how much is the external call going to spend (which is the point of this PR) so essentially we want a placeholder that does that for us, which would pass remaining_fuel() - external_call_fuel_cost

[iFrostizz]

It would be great if this function could only accept units that are > 0, there might be some semantic changes to do

[iFrostizz]

Otherwise, replicate the Gas enum type on the go side too so that when it's the Unit variant then it's only interpreted as "pass x units" even when it's 0 to avoid any runtime panic because of the conversion from a u64 to a NonZeroU64.

[iFrostizz]

We might also want to replace the call_contract function by a function that only accepts the Address, function_name, args.

[iFrostizz]

To keep being explicit on the gas forwarding, having a function named with_max_units(u64) and with_pass_all_gas() would make sense. The default behaviour could be to use Gas::PassAll, so the with_pass_all_gas could be ignored.

[iFrostizz]

Would be interested to know if there could be a cleaner solution. Currently there is a bool and the uint64 that tells the amount of fuel passed, but since having PassAll = true and Fuel > 0 is inconsistent state, this has to be validated through the execution. Is there a better way to simulate this rust enum:

enum Gas {
    PassAll,
    Units(u64)
}

[iFrostizz]

Passing the units of gas to program highest-level calls to the simulator is still required. We might want to set the units of gas in the simulator initialisation but that sounds a little out of scope for this PR.