Add more methods to MultiGas struct

[gligneul]

No description provided.

[eljobe]

@gligneul, are you ready for review? If so, can you click, "Ready for review", give it a proper PR description and assign this to me?
If not, what's still outstanding?

[gligneul]

@gligneul, are you ready for review? If so, can you click, "Ready for review", give it a proper PR description and assign this to me?
If not, what's still outstanding?

There are quite a few things missing. I will open it soon.

[gligneul]

I decided to open this PR with just the multigas methods. Later, I will open another PR passing the multigas variable to the interpreter.

[MishkaRogachev]

Overall everything looks good, there is one small suggestion:

func (mg *MultiGas) ToLegacyGas() uint64 {
   var sum uint64
   for _, value := range mg.gas {
   	sum = math.SafeAdd(sum, value)
   }
   return sum
}

it will help me in the tests

[eljobe]

Overall everything looks good, there is one small suggestion:

func (mg *MultiGas) ToLegacyGas() uint64 {
   var sum uint64
   for _, value := range mg.gas {
   	sum = math.SafeAdd(sum, value)
   }
   return sum
}

it will help me in the tests

I would like to not call such a method ToLegacyGas because it makes it seem like we are no longer using that kind of gas. But, in fact, single-dimensional gas is still a valid concept in the system.
Let's go with SingleDimGas() or SingleGas() or Aggregate() or something like that.