By default have SMAR use nested transactions

[krainboltgreene]

Okay, so this is a first pass at a PR where we invite the developer to opt-in OR opt-out of nested transactions. I started small to see how the maintainers would react to this.

[krainboltgreene]

Any thoughts?

[rafaelfranca]

What is the reason to change this default?

[krainboltgreene]

Whenever I talk to people about SMAR I ask them about transitions and transactions, usually they have two (incorrect) assumptions:

1. SMAR (before|around|after) is not in a transaction
2. A transition (inside of another transition) that raises rollback will rollback changes and also rollback the parent transition

Given that I think it would be really valuable for SMAR to both know about nested transactions (obviously I would have to add more to this PR ) but also act like people expect for things like rollbacks and previous_changes.

I'm willing to put a lot of work into this if you give me the green light.

[rafaelfranca]

That seems reasonable. Let's implement this.

[krainboltgreene]

Awesome, I'll start this weekend with some failing tests and then go from there.