Abstract away wasmi

[lightclient]

Although wasmi has been a great initial interpreter for prototyping, it's unlikely that it will be used for the long run. It's important to keep this in mind as we continue work on the runtime and actively avoid tight coupling between our libraries.

The most pressing concern is wasmi's lack of stack traces. This makes it very difficult to track down bugs in the wasm blobs. I managed to address this issue in sheth by writing two main functions, one that used the ewasm FFI and one that received the same info via CLI args. This worked fine when the FFI functions could sandwich the execution, but with the addition of child runtimes in #11 I don't think that model will continue working.

I haven't done much research into it, but we should checkout other wasm interpreters / compiler systems to see what kind of error handling they provide. Even though the current road map is to use a wasm interpreter for Eth2, I'm not opposed to integrating an optimized compiler to this repo for testing purposes if it can provide robust stack traces.