Add keccak256 host function

[zjg555543]

Support keccak256 native code function.

• keccak256 is a frequently-used function in contracts.
• native code is more efficient than using libraries in contracts.

[omniwired]

I was wondering if @webmaster128 saw this one.

[webmaster128]

Right, so in the past when working on crypto APIs we conclude that all hash function should be executed right in Wasm instead of adding a call to the environment. The reasoning is the following:

• Hash functions are cheap in execution cost and code size
• There is a huge amount of hash functions needed (keccak256, SHA2, SHA3, ripemd160, various flavours of blake, ...)
• Benchmarking and setting a gas price for those functions is a lot of work
• Keeping the contract-host API minimal is good for maintenance

In contrast to EVM environments, Wasm allows for much faster execution in the contract.

We added a few crypto functions that are much heavier than hashing, like secp256k1 and ed25519 verification. Pairing based cryptography might join at some point. For those cases we win a lot in terms of code size and execution speed when having them in the host.