Move Combine's TopLevelEncoder and TopLevelDecoder protocols into the standard library

[beccadax (Becca Royal-Gordon)]

@danielctull

I think for ABI stability reasons, Combine will still need to provide these types.

Thanks to work by @Xi_Ge, we now have (unofficial, compiler-internal, no-user-serviceable-parts-inside, offer-void-in-Nebraska) support for moving declarations from one library "up" into another library that the original imports. Since every Swift library imports the standard library, I think this would make it possible to move these protocols into the standard library without breaking ABI.

@danielctull

However, I believe Combine could re-define TopLevelEncoder to be a typealias for the swift standard library's protocol, which would allow them to interoperate.

A typealias would preserve source stability, but not ABI stability.