execution_set.check_auth(...) on initial subscription

[Centril]

Description of Changes

We simply didn't check auth for initial subscriptions, and this PR fixes that.

API and ABI breaking changes

Technically this breaks the API in the sense that we check auth now, but that is a bug fix...

Expected complexity level and risk

1

Testing

A test is added that ensures that we check auth.

[joshua-spacetime]

Also simplifies fn sources(), using internal iteration rather than external, avoiding some allocation when checking auth.

Bug fixes and refactors really should be separate.

[Centril]

Also simplifies fn sources(), using internal iteration rather than external, avoiding some allocation when checking auth.

Bug fixes and refactors really should be separate.

I don't agree with this in the general case as sometimes you need to refactor to fix a bug in a good way, and sometimes a bug fix falls out of a refactor. For the purposes of a hotfix, I think it's also sufficient to have the fix and the refactor in a separate commit as that allows cherry-picking the former into the hotfix branch. This would be fine e.g., for a rustc beta-backport PR, which is more high risk than anything we are currently dealing with. But in the interest of moving this along quicker, I'll move the second commit to a different PR.

[mamcx]

LGTM