Resolve Swift Async Algorithms import conflicts

[lachenmayer]

Importing AsyncExtensions and Swift Async Algorithms together makes Sequence.async, the fixed two- and three-input zip/merge APIs, and the AsyncTimerSequence type ambiguous. This change delegates .async and the fixed combiners to AsyncAlgorithms, removes their duplicate implementations and unused internal code, and renames AsyncExtensions’ buffered Date timer to AsyncBufferedTimerSequence while preserving its behavior.

Preserve AsyncExtensions’ variadic zip/merge and explicit AsyncLazySequence constructor. Update the remaining tests to import AsyncAlgorithms where needed and explicitly exercise AsyncExtensions’ variadic operators. Add eight coexistence tests covering .async, Apple’s fixed overloads, AsyncExtensions’ four-input overloads, and both timer types. Rebase onto current main, retaining its subject and cancellation fixes.

This is a breaking API migration: callers using the removed APIs must add the AsyncAlgorithms product dependency and import, and callers using the buffered Date timer must adopt its new name. README and changelog include migration guidance. The package now requires Swift 5.8 for the AsyncAlgorithms test dependency; the production target retains only its Collections dependency.

Validation on macOS with Swift 6.4:

• Full suite with coverage: 171 tests passed using AsyncAlgorithms 1.0.0 and Collections 1.0.5.
• Compatibility tests: all eight passed with AsyncAlgorithms 1.1.7 and Collections 1.7.1 in an isolated dependency-update check.
• git diff --check passed.