Fix Xcode 16 build failure caused by AsyncSubject

[gsl-anthonymerle]

Fix AsyncSubject's Failure type conflincting with Apple addition to AsyncSequence starting from Xcode 16 (iOS 18, MacOS 15, Vision OS 2, etc.)

Description

For more information on what caused the issue, see this discussion.

Fixed by:

• Splitting AsyncSubject procotol into two protocols:
  • AsyncSubjectable, meant to define the send(element:) method and dependance to AsyncSequence and other protocols
  • AsyncSubjectTerminable, meant to define both the AsyncSubject's Failure type and the send(termination:) method
• Making AsyncSubject a typealias to a conformance of both of these new protocols

This results in the compiler either:

• using the AsyncSubjectTerminable's Failure type on its own when AsyncSequence doesn't defines it (up until Xcode 16)
• allowing both AsyncSubjectTerminable and AsyncSequence's Failure type to be the same type when AsyncSequence does define its own Failure type (starting from Xcode 16)

Checklist

• this PR is based on the main branch and is up-to-date, if not please rebase your branch on the top of main
• the commits inside this PR have explicit commit messages
• unit tests cover the new feature or the bug fix
• the feature is documented in the README.md if it makes sense
• the CHANGELOG is up-to-date

[twittemb]

gonna review this asap

[gsl-anthonymerle]

gonna review this asap

Sure! Thanks a lot 🙏

[twittemb]

Seems good to me. Thanks a lot.