feat!: make callback opaque

[wyfo]

Use an enum hidden to use channel senders directly. It should improve performances, and may allow later to use async sender methods.

[github-actions[bot]]

PR missing one of the required labels: {'dependencies', 'documentation', 'breaking-change', 'bug', 'enhancement', 'internal', 'new feature'}

[wyfo]

Using a immutable reference as callback argument prevent getting a mutable access to a shared memory segment contained in payload. The solutions would be either to use a mutable reference, or to keep the argument by value. @kydos said he prefer keeping the current semantic, so let's go. I will the PR accordingly.

[DenisBiryukov91]

Using a immutable reference as callback argument prevent getting a mutable access to a shared memory segment contained in payload.

Access by value is still available through handlers if needed.

[Mallets]

Using a immutable reference as callback argument prevent getting a mutable access to a shared memory segment contained in payload. The solutions would be either to use a mutable reference, or to keep the argument by value. @kydos said he prefer keeping the current semantic, so let's go. I will the PR accordingly.

With current semantics you mean passing Sample by value?

[wyfo]

Yes

[wyfo]

Replaced by #1405