Add simple gas metering on PVM

[pantrif]

This PR introduces basic gas metering, where each instruction consumes 1 gas, since a gas cost model has not been defined yet.

Part of this #39

[danielvladco]

could we use a wrapper pattern for the mutator?

we could have gasMutator struct that wraps the mutator and returns another mutator with gas functionality added
for example:

func NewGasMutator(next Mutator) Mutator {
    return &gasMutator{next: next}
}

func (m *gasMutator) Trap() error {
    m.remainingGas -= 1
    return m.next.Trap()
}

// and so on for all the operatios

then we pass it like this

instruction.StepOnce(NewGasMutator(mutator))

This approach might have a bit more boilerplate, but it seems more idiomatic and doesn't require one map lookup for each instruction, which could add up quite a lot

[pantrif]

could we use a wrapper pattern for the mutator?

we could have gasMutator struct that wraps the mutator and returns another mutator with gas functionality added for example:

func NewGasMutator(next Mutator) Mutator {
    return &gasMutator{next: next}
}

func (m *gasMutator) Trap() error {
    m.remainingGas -= 1
    return m.next.Trap()
}

// and so on for all the operatios

then we pass it like this

instruction.StepOnce(NewGasMutator(mutator))

This approach might have a bit more boilerplate, but it seems more idiomatic and doesn't require one map lookup for each instruction, which could add up quite a lot

I understand your point, but the main reason for using a map is to have a centralized, easily modifiable place once the cost model is defined. However if you have a strong opinion about it i will change it.

[danielvladco]

LGTM