Introduces event classes, like Events::Create, which wrap the
AR instance and have one public method, `#data`, which returns
the attributes of nascent `Version` records. See comments in
Events::Base.
This extraction greatly simplifies RecordTrail. Not only have
we moved a lot of code out of RecordTrail, but we've also
moved the `@in_after_callback` variable. In its new location, this
variable is much easier to understand, and we no longer need the
`ensure` blocks to reset it.
Preserving compatibility with PT-AT caused some minor difficulties,
and we should follow up with them to see if we can simplify the
API they are using. Specifically, we would like to delete the
`data_for_*` methods.
The reason some projects have both is so that some spec files can be
run in isolation, without the rails stuff. In practice, I don't find
myself ever doing this. So, the complexity of two files is unnecessary.
Rumble Huang