Proposal: Deprecate decodeUtf8

[Henning Thielemann]

On Mon, 22 Aug 2016, Pavlo Kerestey wrote:

> In my opinion decodeUtf8 is an example of "there is nothing obvously 
> wrong" with it. I think I was lazy to copy it from StackOverflow once or 
> twice as a solution to `ByteString -> Text` but have not checked the 
> docs. I was just not careful and forgot that it is not so pure.

It is pure (in the sense of purely functional), but it is not total.