eth/tracers: add multiplexing tracer

[s1na]

Sometimes users might need the result of multiple tracers for a given tx. This PR introduces the muxTracer which allows you to do this. It takes in a list of tracers and their configs and returns each one's corresponding result with one run through the tx:

> debug.traceTransaction('0xf8fd7bea3dc42ee81922584884d3f741e8e475ecccb75b0e4f2dad042061c723', { tracer: 'muxTracer', tracerConfig: { prestateTracer: { diffMode: true}, callTracer: null }})
{
  callTracer: {
    from: "0x2806a6d83218acc8b223063bd058957d38966a09",
    gas: "0x0",
    gasUsed: "0x5208",
    input: "0x",
    to: "0x2806a6d83218acc8b223063bd058957d38966a09",
    type: "CALL",
    value: "0x1"
  },
  prestateTracer: {
    post: {
      0x2806a6d83218acc8b223063bd058957d38966a09: {
        balance: "0xffffffffffffffffffffffffffffffffffffffffffffffffffffef49bca6f9f7",
        nonce: 1
      }
    },
    pre: {
      0x2806a6d83218acc8b223063bd058957d38966a09: {
        balance: "0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff7"
      }
    }
  }
}

There's a trick that allows you to use the same tracer with different configs by using a nested muxTracer (e.g. to get both prestate and statediff):

> debug.traceTransaction('0xf8fd7bea3dc42ee81922584884d3f741e8e475ecccb75b0e4f2dad042061c723', { tracer: 'muxTracer', tracerConfig: { prestateTracer: { diffMode: true}, muxTracer: { prestateTracer: null } }})
{
  muxTracer: {
    prestateTracer: {
      0x2806a6d83218acc8b223063bd058957d38966a09: {
        balance: "0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff7"
      }
    }
  },
  prestateTracer: {
    post: {
      0x2806a6d83218acc8b223063bd058957d38966a09: {
        balance: "0xffffffffffffffffffffffffffffffffffffffffffffffffffffef49bca6f9f7",
        nonce: 1
      }
    },
    pre: {
      0x2806a6d83218acc8b223063bd058957d38966a09: {
        balance: "0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff7"
      }
    }
  }
}

It's possible to directly add support for this to the API, e.g. debug.traceTransaction('<txhash>', { tracerList: {...} }) but I feel like being able to add this feature without having to modify the API is a big plus.

Unrelated commit in the PR: I also moved EVM cancellation to be the API's responsibility instead of the tracer. I feel like it's more fitting.

[rjl493456442]

Good idea!

[holiman]

This is a nice idea. I'm curious to see if it is "only" a nice idea, or if it actually is meaningful too, in the sense that people will use it.

Up until now, I've been kind of starting to see a tendency to make tracers more complex, adding tracer-specific configurations (e.g. the call-tracer taking a config whether log log LOGs etc). This PR might make it a bit more "the Unix way", so the tracers are an assortment of small simple tools, and the ability to mux makes it possible to avoid complex tracers with everything and the kitchen-sink.