Normative: Fix state checking in async cycle case

[littledan]

If a cycle contains async modules, then its statuses will not be set
to "evaluating-async" until after the cycle is traversed. This could
cause errors in setting up the async dependency graph edges. This PR
uses different checks to avoid the issue.

Fixes #89

[guybedford]

This is nicer than using "evaluating-async" for this check. The only thing I would add is that we have three "evaluating" checks through the steps here, which might be able to be combined in some way, but that's entirely editorial.