0.12.0

This release adds a new OpenCombineShim product that will conditionally re-export either
Combine on Apple platforms, or OpenCombine on other platforms. Additionally, ObservableObject
protocol is now available and working on all platforms.

A bug with Timer(timeInterval:repeats:block:) firing immediately not accounting for the passed
timeInterval is fixed.

Merged pull requests:

• Fix Timer(timeInterval:repeats:block:) not accounting timeInterval (#196) via @grigorye
• Add OpenCombineShim product for easier importing (#197) via @MaxDesiatov
• Implementation for ObservableObject with Mirror (#201) via @kateinoigakukun