Our OpenCombine fork no longer depends on Runtime, and we don't need much from it other than struct metadata. I removed the unused bits and bobs and kept only a minimal subset of it that we really need. This should make it easier for us to test and debug, as #367 has shown that some weird stuff may still lurk in that area.
* Add a test for environment injection
We had some issues in this code area previously and I'm thinking of refactoring it in attempt to fix#367. Would be great to increase the test coverage here before further refactoring.
* Update copyright years in `MountedElement.swift`
* Update copyright years in the rest of the files
* Vend the Runtime library directly
* Remove unused class, enum, tuple, func reflection
* Remove unused models and protocol metadata
* Remove unused MetadataType and NominalMetadataType
* Remove unused protocols, rename RelativePointer
* Remove more unused protocols
* Use immutable pointers for reflection
* Update copyright headers
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