…ng preimages

This changes makes it so that the block test executor takes a callback, just like
the state test executor already does. This callback can be used to examine the
post-execution state, e.g. to aid debugging of test failures.

This change also modifies the mechanics of Dump: previously, all accounts for which
we did not have the preimage (address) were silently discarded.

Now they are output instead like this:
```
{
    "root": "3ef447699d08e3160c448903e77bd092c67ce3faeba773a3f83c500ab80f2400",
    "accounts": {
        "pre(0x03601462093b5945d1676df093446790fd31b20e7b12a2e8e5e09d068109616b)": {
            "balance": "10780210",
            "nonce": 1,
```