Fix return checking in behaviours and constructors

[SeanTAllen]

Our checks to make sure that the usage of return in behaviours and constructors was overly restrictive. It was checking for any usage of return that had a value including when the return wasn't from the method itself.

For example:

actor Main
  new create(env: Env) =>
    let f = {() => if true then return None end}

Would return an error despite the return being in a lambda and therefore not returning a value from the constructor.

Fixes #3682

[SeanTAllen]

To be a full PR, this needs a regression test added as well as release notes and the PR comments etc cleaned up.

[ponylang-main]

Hi @SeanTAllen,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 3971.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

When this is squashed for merging, the comment should be:

---

Fix return checking in behaviours and constructors

Our checks to make sure that the usage of return in behaviours and constructors was overly restrictive. It was checking for any usage of return that had a value including when the return wasn't from the method itself.

For example:

actor Main
  new create(env: Env) =>
    let f = {() => if true then return None end}

Would return an error despite the return being in a lambda and therefore not returning a value from the constructor.

[jemc]

Looks good, but can you also add a test showing that this works for object literals (and if it doesn't, fix the code so that it does)?

[SeanTAllen]

@jemc 2 additional tests were added. Let me know if it looks good. This always worked for object literals. The problem was the check came before the lambda's got desugared. The additional tests verify that it working for object literals (an axiom upon which this is based) continues to remain true.