Simplifying sequential() / parallel()

[Brian Goetz]

> Stateful programming has its issues but that ship has already sailed (in
> Java).

While that's unquestionably true, I think it is also unnecessarily 
defeatist.  A tremendous amount of effort has gone into the design of 
this API to make statefulness less attractive because there's an easier 
way to do it without statefulness.

> I don't want to create danger zones in the code where these
> transformations are accidents waiting to happen. Also think of the code
> maintainers trying to determine, as they are enhancing and debugging the
> code, where they are allowed to add state.
>
> Before, the existence of parallel() created a danger zone, but
> sequential() restored safety. That's an easy rule to understand.

The newly proposed rule does the same.  The only problem is what happens 
when responsibility for a pipeline is divided across code regions. 
Which I'm arguing is always problematic with statefulness lambdas 
regardless of model.