fix: rewrite OrDone() to be more clear and make sure it handles all data

[computator]

Currently if OrDone() receives data at the same time as a done signal it could potentially quit before sending on that last data. This resolves that race condition as well as rewriting the code to make the process more clear.

[computator]

Closing this per discussion:

• Although it might be theoretically possible to receive on in and done at the same time, that is not relevant for how OrDone() is intended to be used.
• Although the rest of the rewrite may be more detailed in how the process works, it is less idiomatic.