Rewrite async page+spread operators for native promises

[vitaly-t]

Async operators page and spread should be easy to replace with promise-only implementation, for maximum performance.

Review the rest of async syntax.

[vitaly-t]

This has been done and released in version 1.4.8

Remaining async operators that still use async/await syntax:

• concat
• defaultEmpty
• repeat
• split

That's 4 out of 35 total operators, not bad.

Perhaps repeat is the only one worth revisiting later, while the rest are just too big, and rewriting them into native promises won't bring much benefit, if any.