Implement metering in wasmer-python

[brunokim]

I've been writing a silly compiler from Whitespace to WASM, and I'm testing it using Wasmer.

I've been trying to root out bugs by fuzzing inputs, but this presents a problem when the fuzzer finds an infinite loop. Using metering seems the best option here, but I see that it's not available in the Python interface.

I don't have any experience with Rust, but skimming the codebase, it seems it wouldn't be too hard to surface this feature for Python. Do you have any tips or design considerations before I jump into this? I'd like to have it available in mainline if you're accepting push requests.