state: add new state package as a foundation for v2

[ecordell]

Description

There's lots of docs in the PR so I'll let those do most of the talking, but there were a few pain points I set out to address with this:

1. It was too easy to forget to return from a handler, i.e. after calling requeue. This new state package relies on returning the next step to run (vs. directly calling the next step to run like in v1), so you can't forget. This is the primary low-level difference - building blocks are func(Context) Context instead of func(Context). There are some new things in the queue package to demonstrate this use too.
2. I really disliked the fact that handlers needed string names in v1. The main reason was so that we could "find" the right handler if we needed to branch. The new state package addresses this by giving direct ways to compose handlers (examples will make it clear).

And in the back of my head I've always thought about these in terms of monadic structures. I don't think it's for everyone, but I took the time to write down what that actually means in the context of the state package and why it matters (library consumers don't need to worry about this, it's more philosophical)

Testing

This is not in practical use yet - there are unit tests and and I have follow on work that uses these new primitives, but wanted to keep the initial PR smaller.

[tstirrat15]

See comments

[tstirrat15]

Or is the idea that we're using the error field on the context for this? Like where would this be set other than through cancellation?

[ecordell]

this is just for handling context cancellation, we do it a bunch so I wanted to make some helpers for it

[tstirrat15]

General comment: I still don't understand how error handling works in this - how would an error result of (say) an API call be propagated up to an error-handling step? Are we sticking it on the context? That seems like it'd be a recipe for unhandled errors or error handling that behaves strangely when you introduce parallelism.

[ecordell]

#83 (comment)

yeah I addressed that here - I think I need a bit more docs / examples to explain it

[ecordell]

I've simplified and cleaned up a bit since I opened this, the new things to note are:

• NewTerminalStepFunc helper - convenience for steps that run a side-effect and terminate
• WithErrorHandler / Continue - error handler mechanism for cancelled contexts, replacing the original auto-recover behavior. Context cancellation now routes through Continue which checks for a registered error handler
• When — sugar for Decision(pred, handler, Noop), one-branch conditional
• Recover / PanicError - explicit opt-in panic recovery that routes the panic through WithErrorHandler rather than implicit recovery in all parallel branches
• ParallelWith - helper to apply a uniform wrapper (e.g. Recover) to all parallel branches