Return data for precompiles

[OlivierBBB]

Here's one way to catch the return data of precompiles:

• add a bit to the java implementation of the hub, say isPrecompileCall; by default it should be set to false
• during tracePreOpCode() you set it like so

if (opCode.isCall()) {
    isPrecompileCall  = calleeAddress.isPrecompile()   // not sure about the syntax
}

• then in tracePostOpcode you do the following

if (isPrecompileCall) {
    frame.setReturnData( [besu's return data] );    // not sure how to get this return data but it should be available through best
}

isPrecompileCall = false;

This leaves open the question of extracting besu's return data (and whether or not it will be available before tracePostOpcode is called.