Add conditional regions

[Kixiron]

Allows the making and using of optional regions, regions that either act like Scope::region() or like nothing at all depending on the parameters given to it, allowing for applications to have their cake and eat it too use regions for dataflow debugging when necessary while still being able to disable them to avoid the overhead incurred by them

Closes #392, unblocks differential-datalog/#990

[Kixiron]

Any updates on conditional regions, I still need this for differential datalog

[frankmcsherry]

I'm hesitant to merge this as is, as it is fairly invasive around very sensitive code. I'd much prefer to prototype, and either succeed with or better understand, a user-level type that allows one to wrap either a stack of regions, or nothing. If that ends up being substantially more trouble than doing it in core timely itself, we can revisit.

[Kixiron]

I'm not sure what you mean by a user level type, timely doesn't expose the required things to create custom subregions so I'm not sure how that'd work. Additionally, I don't actually change anything in pre-existing code, the conditional region is just an additive thing