Adapt to wasmi::ResourceLimiter, replacing mem_fuel metering

[jayz22]

What

This PR adapts to the new wasmi::ResourceLimiter api, replacing our current metering-based guest memory limiting mechanism. The main benefits are:

• More resources (table, instances) are limited, not just memory
• It also limits memory allocation on startup, preventing guest contract from declaring large memory in in the wasm file. Resolves [test] Ensure data segment and user-defined memories can't exceed memory budget limit #896

Why

[TODO: Why this change is being made. Include any context required to understand the why.]

Known limitations

[TODO or N/A]

[jayz22]

I've added two new limits here. I think the linear memory size 2MB should be reasonable? Not sure about the table elements count. Would like some feedback.

[graydon]

While I suspect this all works, I think I might have miscommunicated the approach I had in mind. I was hoping (and please correct me if this is not possible) to make Host impl ResourceLimiter directly, such that the callbacks would deduct memory usage from the Budget directly. I.e. I was thinking we would not be using the StoreLimits type from wasmi -- that is a convenience type that implements ResourceLimiter but it is not the only impl possible and, in our case, not the one I was expecting us to use.

(Part of the reason I want to do it this way is that I would like the total memory budget to be available "in any combination" to all VMs that get instantiated, such that 1 VM can use the entire 100MB of the budget, if it wants, or the 100MB can be split across multiple VMs as they call one another).

[jayz22]

Closing in favor of #950