Native execution of wasm files

[mrsmkl]

What is blocking this at the moment:

• verified initialization: TrueBitFoundation/ocaml-offchain#39
• the result will have to be just the hash of output, when the challenge is posted, the solver will have to submit the initial and final state with proofs that they are compatible with the posted initial code, input and output
• also have to post the number of steps at that time