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Use CXShim instead of hardcoded Combine implementation

22

See [Combine Compatible Package](https://github.com/cx-org/CombineX/wiki/Combine-Compatible-Package) ## Advantage - Current clients remains unaffected. - Apple's Combine is used by default whenever possible. No additional dependency is introduced. - OpenCombine is still available....

Subject should not invoke receiveValue on multiple threads at the same time

1

Failing test case here: https://github.com/cx-org/CombineX/blob/master/Tests/CXInconsistentTests/FailingTests/FailingSubjectSpec.swift

`Publishers.FlatMap` should request `.unlimited` demand instead of `.max(1)` if itself received unlimited demand. Failing test case here: https://github.com/cx-org/CombineX/blob/master/Tests/CXInconsistentTests/FailingTests/FailingFlatMapSpec.swift

The following code crashes, because the matched string breaks extended grapheme cluster. The [range of swift string](https://github.com/sharplet/Regex/blob/master/Source/Regex/MatchResult.swift#L94) is invalid. ```swift let source = "\r\n" let regex = Regex("\n") let match...