Introduce mapError function

[clive819]

Post: https://forums.swift.org/t/pitch-map-error/75422

Pretty much the same as Combine.Publisher.mapError but for AsyncSequence.

This enables you to do something like this:

letsequence= myAsyncSequence.mapError{MyGenericError(wrappedError: $0)}

[phausler]

This needs a pitch document and post.