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

[itaiferber]

[Pitch] Move TopLevelEncoder and TopLevelDecoder to the standard library

Wouldn't you like to use TopLevelEncoder and TopLevelDecoder without importing Combine? I know I would! TopLevelEncoder and TopLevelDecoder are protocols that describe the encoders and decoders engineers use to kick off encoding/decoding an object graph (rather than the local any Encoder and any Decoder values passed to encode(to:) and init(from:) respectively). These protocols are particularly useful for library code that wants to encode and decode values but doesn't really need to care much a…