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