Keccak256 support

[lrettig]

Brought in the keccak256 code from https://github.com/ewasm/evm2wasm/blob/master/wasm/ but I haven't yet figured out how to link it in. For now it lives as an extern in the "ethereum" namespace, but that won't work unless we want to add it as an EEI method.

Opened AssemblyScript/assemblyscript#194 to look into how to do the linking against a wast/wasm in AS.