Add scratchpad to evmc_result

[chfast]

The current size of evmc_result is 64 bytes. There also exists additional API https://github.com/ethereum/evmc/blob/master/include/evmc/helpers.h#L211 that allows to use last 24 bytes for other purposed in case .create_address is not used. This allows to put a well implemented std::vector there.

I propose to remove such usage. Using unions and same fields for different purposed seems to risky. Instead, we should add 64 bytes of scratchpad in the end. This will allow current optimizations (memory stealing) but also using the space for short outputs. I expect in practice the most common output size is 32 for solidity methods returning uint256. Then outputs of size <= 64 can be copied directly to evmc_result's scratch pad.

The problem will get complicated with #413 where additional int64_t field must be added to evmc_result.