Add example of execution limits using fuel consumption

[ryanpbrewster]

This is a quick example of using the "fuel consumption" logic introduced in #2611.

It computes larger and larger Fibonacci numbers (using the expensive recursive algorithm) until the computation requires more than 10k fuel.

[alexcrichton]

Thanks for this!

[alexcrichton]

Ah it looks like CI is failing when it's running the examples, as it expects that all examples pass successfully. Could the example be updated to not return a failure code? (e.g. catch the error)

Also I think our CI verifies that all examples are available in both C and Rust, so I think that at least a stub program will be needed for a C example?

[ryanpbrewster]

Ah, that could be it. Thanks for the pointer, adding a C stub now.

EDIT: This may take a bit, figuring out how to build and run the C examples on a mac now.

[ryanpbrewster]

Added a C example. I'm not particularly confident in the error handling and cleanup code there. I think I handle the error codes from wasm_store_add_fuel (I manually checked that if the config does not enable fuel consumption that those calls fail and exit).

I'm not sure about the semantics of the bool that wasm_store_fuel_consumed returns, so I just ignored it. Should I be asserting that it's true?

[alexcrichton]

That all looks great to me, thanks!