evm/vm: update event signatures

[jochem-brouwer]

If you listen to an event in evm or vm asynchronously, then you need to call a continuation function once you are done;

vm.on('afterTx',  async (e, cont) => {
... do async stuff
cont()
})

However, our type signatures stated that this cont function should always be called with an argument, which is not the case.

[codecov]

Codecov Report

Merging #2184 (d2ce4bf) into master (482caf9) will increase coverage by 0.00%.
The diff coverage is n/a.

Flag	Coverage Δ
block	93.07% <ø> (ø)
blockchain	88.82% <ø> (ø)
client	86.67% <ø> (ø)
common	98.09% <ø> (ø)
devp2p	92.30% <ø> (+0.05%)	⬆️
ethash	∅ <ø> (∅)
evm	62.97% <ø> (ø)
rlp	?
statemanager	88.71% <ø> (ø)
trie	89.27% <ø> (ø)
tx	97.98% <ø> (ø)
util	93.14% <ø> (ø)
vm	85.27% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

[acolytec3]

LGTM