Make sure to propagate a promise job's snapshot when handling abrupt completions

[andreubotella]

Closes #35.

[jridgewell]

With this change, I don't think we need HostCallJobCallback at all. And if we land on something like #16, we'd be able to remove HostMakeJobCallback and instead store directly on the PromiseCapability.

[andreubotella]

With this change, I don't think we need HostCallJobCallback at all. And if we land on something like #16, we'd be able to remove HostMakeJobCallback and instead store directly on the PromiseCapability.

True, I hadn't realized these promise jobs were the only users of HostCallJobCallback (other than FinalizationRegistry callbacks, which I guess should also be considered re. which context to use). I've now changed the PR to swap the context before calling HostCallJobCallback, and to not change that host hook at all.

I don't think it's yet fully clear which option to go for in #16, so I think getting rid of HostMakeJobCallback should be left for a follow-up PR when that is more settled.

[jridgewell]

Need to update CleanupFinalizationRegistry, otherwise LGTM

[jridgewell]

Rebased this and resolved merge conflicts.