DecodingError type conflicts with Foundation's DecodingError

[patrickfreed]

The DecodingError type included in this library shares a name with the DecodingError from Foundation, requiring users of both libraries manually disambiguate. This is particularly noticeable when importing ExtrasBase64 into an existing codebase, since all existing references to Foundation's DecodingError need to be updated in order to compile. Ideally, the type introduced in this library could use a unique prefix to ensure this clash doesn't occur (e.g. ExtrasBase64DecodingErrror).