MSVC warning C4459: declaration of 'cleanup' hides global declaration

[bobbymcr]

When using libunifex with MSVC 19.34.31935 and level 4 warnings enabled, I'm seeing this warning:

spawn_future.hpp(736,29): warning C4459: declaration of 'cleanup' hides global declaration
stream_concepts.hpp(77,5): note: see declaration of 'unifex::_streams::cleanup'

If we really want the stream cleanup to be a global declaration we should avoid using the name cleanup elsewhere.

[bobbymcr]

Seeing similar errors in other places as well:

await_transform.hpp(180,26): warning C4459: declaration of 'sender' hides global declaration
sender_concepts.hpp(192,3): note: see declaration of 'unifex::sender'

[bobbymcr]

scheduler_concepts.hpp(224,10): warning C4459: declaration of 'scheduler' hides global declaration
scheduler_concepts.hpp(101,3): note: see declaration of 'unifex::scheduler'