Migrate AsyncSeq<'T> to BCL System.Collections.Generic.IAsyncEnumerable<'T> (v4.0)

BREAKING CHANGE: AsyncSeq<'T> is now an alias for
System.Collections.Generic.IAsyncEnumerable<'T>, replacing the library's
own IAsyncEnumerator<'T> / IAsyncEnumerable<'T> pull-based interfaces.

Key changes:
- Remove public IAsyncEnumerator<'T> and IAsyncEnumerable<'T> interfaces.
  Code with custom enumerator implementations must be updated.
- AsyncSeq<'T> = System.Collections.Generic.IAsyncEnumerable<'T>
- Internal machinery: IAsyncSeqEnumerator<'T> (pull-based) retained as an
  implementation detail wrapped by AsyncSeqImpl<'T> adapter; not exported.
- AsyncSeqEnumeratorExtensions module: adds GetEnumerator() extension on
  IAsyncEnumerable<'T> for internal use (hidden by .fsi).
- ofAsyncEnum / toAsyncEnum: now identity functions (no-ops).
- Bump version to 4.0.0-alpha.1.
- Update test that used old GetEnumerator()/MoveNext() API to BCL style.

All 190 tests pass.

Fixes #230.

Co-authored-by: Copilot <[email protected]>

Author: 2 people