runevm: evm interpreter on ewasm

[s1na]

Depends on #431.

Summary runevm compiles parity's evm interpreter to wasm, and uses the ewasm-rust-api to communicate with the Ethereum environment. This PR extends the EEI to allow running simple evm bytecodes on top of ewasm through runevm. It's currently passing the vmArithmeticTest suite :) To run the tests use npm run testRunevm. The good point about VMTests is that they don't explicitly require the state trie. So to simulate the state I use a simple in-memory mock object, and avoid the sync/async problem for now.

Notes:

• Not to be merged, this is an experiment with two purposes:
  1. serve as a pipline for testing runevm
  2. incrementally fill in more ewasm features in ethereumjs-vm and figure out challenges
• The runevm contract was built from the Consume gas before finish axic/runevm#9 branch
• As in Basic support for ewasm precompiles #431, this PR is heavily influenced by ewasm-kernel

[coveralls]

Coverage decreased (-2.5%) to 92.589% when pulling b5eb9fd on ewasm/runevm into 690f5f9 on ewasm-precompiles.

[holgerd77]

Same here, do we want to close here?