`ResourceBoundsMapping` internal repr

[tdelabro]

ResourceBoundsMapping only contains two values for now, L1Gas and L2Gas.

First of all, why is it represented as a BTreeMap? It could just be a struct with two ResourceBounds fields. Is it to be future-proof? That's an unnecessary worry and it comes with performance cost.

Even if we keep it like this, could we at least implement some getters function on ResourceBoundsMapping:

fn l1_gas(&self) -> &ResourceBounds;
fn l2_gas(&self) -> &ResourceBounds;

It can also return an Option<ResourceBounds> if a tx without one or both values is deemed valid.

I would avoid having to write this

self.resource_bound.0.get(resource)