Add evm tracing debug method

[axic]

Similar to the evmStackTrace method in ewasm-kernel: https://github.com/ewasm/ewasm-kernel/blob/master/debugInterface.js#L30-L40

It should target to have all the content needed for the tracing format as described here: ethereum/tests#249

I propose the following: evmTrace(pc: i32, opcode: i32, cost: i32, sp: i32)

It will then output the following JSON: {'pc': $pc, 'op': $opcode, 'gas': $gasLeft, 'gasCost': $cost, 'stack': $stack, 'depth': $depth} where $gasLeft and $depth are the current values from the environment and $stack is an array of reading 32 bytes from memory_start + $pc and decreasing $pc by 32 until it is less than 0.

[axic]

cc @cdetrio what do you think?

This should be close enough to both what ewasm-kernel does and what the tracing JSON requires.