RxSwiftExt 5.1.0

• RxSwiftExt now supports Swift Package Manger (SPM)
• Added UIScrollView.scrolledToBottom
• Fix deprecated barrier usage in once
• Added merge(with:)
• Simplify unwrapping operators to use compactMap underneath the hood.
• Add mapTo for Traits.
• Make pausableBuffered more robust to reentrancy.