Enhancement: Add flattedEvents selector to debugger

[haltman-at]

This PR adds to the debugger the seletor txlog.views.flattedEvents. This selector gives a flat list of all events emitted during the transaction so far; note that as a txlog selector it's meant primarily to be used at the end of a transaction.

The selector's return value is an array of all events emitted during the transaction so far, in order; note, however, that this includes events that may have reverted. However, each event includes a status field to distinguish, so if you want you can filter down to just the events that were successfully emitted (and then you can put indices on them if you really want :) ). (I thought about adding an additional selector that actually performed this filtering, but this seemed unnecessary so I skipped it.) Note that if you look at this selector before the transaction has completed, then status: true will merely indicate that the event hasn't reverted yet; it may still revert in the future.

Each event contains the fields decoding, raw (with topics and data), address, codeAddress, and status. The decoding is the same decoding from txlog; raw is the same as well. The address field lists the address that officially emitted the event, while codeAddress is the address of the actual code that emitted the event; these may differ in the event of a delegate call (as it's the storage address, not the code address, that officially emits the event). Note that if a constructor emits an event, the address being constructed is used for both address and codeAddress. Also note as usual that if a constructor reverts, we may not be able to determine the address, and it may show up as zero instead.

I'll skip explaining how it works as it's just a straightforward recursive traversal of the txlog tree. Note that an event will be marked as status: false if either the call that made it reverts, or if any containing call reverts, because either way it was not successfully emitted!

I also added a test.